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Single-Atom Catalysis Hot Paper

Deutsche Ausgabe:
DOI: 10.1002/ange.201805820
Internationale Ausgabe: DOI: 10.1002/anie.201805820

Design of Single Gold Atoms on Nitrogen-Doped Carbon for
Molecular Recognition in Alkyne Semi-Hydrogenation
Ronghe Lin+, Davide Albani+, Edvin Fako, Selina K. Kaiser, Olga V. Safonova, NÇria LÛpez,*
and Javier Pÿrez-RamÌrez*
Abstract: Single-atom heterogeneous catalysts with welldefined architectures are promising for deriving structure–
performance relationships, but the challenge lies in finely
tuning the structural and electronic properties of the metal. To
tackle this point, a new approach based on the surface diffusion
of gold atoms on different cavities of N-doped carbon is
presented. By controlling the activation temperature, the
coordination neighbors (Cl, O, N) and the oxidation state of
the metal can be tailored. Semi-hydrogenation of various
alkynes on the single-atom gold catalysts displays substratedependent catalytic responses; structure insensitive for alkynols with g-OH and unfunctionalized alkynes, and sensitive for
alkynols with a-OH. Density functional theory links the
sensitivity for alkynols to the strong interaction between the
substrate and specific gold-cavity ensembles, mimicking
a molecular recognition pattern that allows to identify the
cavity site and to enhance the catalytic activity.

Identifying the structure of the active site of a catalyst is

pivotal for rational design. While the architecture of homogeneous systems can be easily characterized, the analogue in
heterogeneous catalysis remains very challenging due to the
inherent material complexity, even though advanced in situ
and operando techniques are nowadays widely available. For
this reason, synthesis of heterogeneous metal catalysts with
well-defined active ensembles to mimic organometallic complexes is regarded as an elegant approach, which might not
only enable selective transformations,[1] but exquisitely lead
to the establishment of new structure–performance relation[*] Dr. R. Lin,[+] D. Albani,[+] S. K. Kaiser, Prof. J. Pÿrez-RamÌrez
Institute for Chemical and Bioengineering
Department of Chemistry and Applied Biosciences
ETH Zurich
Vladimir-Prelog-Weg 1, 8093 Zurich (Switzerland)
E-mail: jpr@chem.ethz.ch
E. Fako, Prof. N. LÛpez
Institute of Chemical Research of Catalonia (ICIQ)
The Barcelona Institute of Science and Technology
Av. PaÔsos Catalans 16, 43007 Tarragona (Spain)
E-mail: nlopez@iciq.es
Dr. O. V. Safonova
Paul Scherrer Institute
5232 Villigen PSI (Switzerland)
[+] These authors contributed equally to this work.
Supporting information (catalyst preparation, characterization, and
evaluation as well as computational methods) and the ORCID
identification number(s) for the author(s) of this article can be found
under:
https://doi.org/10.1002/anie.201805820.

514

ships.[2] Single-atom heterogeneous catalysts (SAHCs), a class
of burgeoning materials with diverse applications in chemical
transformation[2a,c] and energy conversion,[3] can be envisaged
as the missing link between homogeneous and heterogeneous
systems owing to the intrinsic cationic nature and well
isolation of the single metal atoms entrapped in a solid
material acting as ligand. To overcome their tendency of
metal agglomeration, extensive efforts have been devoted to
developing effective synthetic strategies via either introducing suitable anchoring sites or creating strong metal-support
interactions, as recently well documented for a wide range of
transition metals on different hosts.[4] Despite being successfully applied in different reactions, still the rational design of
SAHCs for targeted applications is at the initial stage.
Consequently, this leads to a knowledge gap whether concepts
belonging to homogeneous catalysts could be transposed to
the heterogeneous counterparts.
Catalysis by gold has witnessed exceptional growth in
diverse fields since this metal is both effective as a homogeneous or a heterogeneous catalyst.[5] In view of the nowadays
growing importance of gold nanoparticles and single atoms in
hydrogenation,[6] oxidation,[7] water-gas shift,[2a,b] and hydrochlorination,[8] an in depth study on the precise control over
the metal oxidation state and coordination sphere of single
gold atoms using nitrogen-doped carbon (NC) is targeted.
This is achieved by inducing surface diffusion of metal atoms
on the diverse anchoring sites of the host, which leads to the
creation of a series of single-atom gold ensembles with welldefined structures and electronic properties. Inspired by the
opposite origins for supported gold nanoparticles and homogeneous gold complexes governing the selective activation of
alkynes (a phenomenon known as alkynophilicity[9]), the Au
SAHCs with precise architectures are assessed in the semihydrogenation of alkynes to establish catalyst design rules.
Gold was deposited via incipient wetness impregnation of
HAuCl4 in an aqua regia solution either on sulfur-free
polyaniline-derived high N-content NC (ca. 10 wt %)[10] or
activated carbon (AC, as a reference) followed by thermal
activation at different atmospheres and temperatures (413–
1073 K) for 16 h (Figure 1, see the Supporting Information for
more details). The temperature-dependent evolution of Au
particle size for both series was followed by X-ray diffraction
patterns (XRD). Apart from showing the amorphous nature
of the NC support and the absence of any Au related
diffraction patterns on Au/NC-T (where T stands for the
treatment temperature applied), it reveals the characteristic
peaks of the Au(111) facet at T 573 K on the AC reference
(Figure S1). The diverging evolution of the gold atoms on
both carriers was further analyzed by combining high-angle

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Figure 1. Synthetic steps involved in the preparation of nitrogen-doped
carbon (NC) with a high density of electron-rich cavities: oxidative
polymerization of aniline with ammonium persulfate to polyaniline and
subsequent carbonization of the polymer at 1073 K, and the subsequent preparation of supported single-atom gold catalysts via incipient
wetness impregnation of NC with HAuCl4 in aqua regia solution,
followed by thermal activation step in static air ( 473 K) or flowing N2
(> 473 K) for 16 h. Au yellow, C gray, N blue, and Cl green.

annular dark-field scanning transmission electron microscope
(HAADF-STEM, Figure 2 a,b, Figures S2 and S3 in the
Supporting Information) and extended X-ray absorption
fine structure spectroscopy (EXAFS, Figure 2 e), confirming
that the gold species are present as single atoms on the NC
carrier even up to 1073 K. In stark contrast, single Au atoms
hosted on the AC are stable only at 413 K and dramatic
agglomeration occurs readily at T 573 K, featuring scattering peaks assigned to the Au-Au bonds in the Au/AC-673
sample (Figure 2 e) and very wide particle size distribution
(Figure S4).
To assess the individual roles of aqua regia and the N sites,
an additional Au/NC-413(H2O) sample was prepared using
deionized water as solvent. The XRD patterns of the sample
dried at 413 K show clear reflections of metallic gold (Figure S1). This result together with the diverging evolution of
gold on both carbon hosts highlights the important roles of
the solvent and N dopant in stabilizing single-gold atoms on
the carbon host (details are given in the Supporting Information).
To reveal the nature of the gold species on Au/NC-T and
Au/AC-T, X-ray photoelectron spectroscopy (XPS) was
conducted. Interestingly, for the NC series the Au 4f core
level spectra steadily shift to higher binding energy (BE) upon
increasing activation temperature (Figure 2 c). This behavior
is not observed on the AC references, as evidenced from the
similar Au 4f7/2 peaks at BE = 84.2 eV for metallic Au on both
Au/AC-413 and Au/AC-673. Detailed XPS fitting for Au/NC
samples indicates that the low-temperature sample contains
predominantly Au species at BE = 84.2 eV which decrease in
population with increasing temperature, while the contents of
Au+ (BE = 85.0 eV) and Au3+ (BE = 87.0 eV) increase. At
T = 1073 K, Au3+ was the only species stable at such a high
temperature. Thus, the well-behaved temperature dependency of the Au oxidation state enables ad hoc tuning of the
Au/NC system for targeted applications.
The precise assignment of the low BE peak should be
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sensitive and can be reduced to Au0 during XPS measurement.[11] To complement the XPS findings, X-ray absorption
near edge spectroscopy (XANES) was applied as a nondestructive technique for these sensitive materials. Figure 2 d
presents the XANES spectra of relevant samples together
with reference gold standards. For the two extreme Au/NC
samples (Au/NC-413 and Au/NC-1073) and Au/AC-413,
a clear white line peaked at approximately 11 919–11 920 eV
was observed. Besides, the absorption threshold of gold,
obtained from the first derivative of the normalized absorption curves (Figure S5), shows a clear shift to higher energy
for the Au/NC samples as compared to the Au foil, indicating
the cationic gold speciation in these materials.[12] On the
contrary, the Au/AC-673, containing mainly Au nanoparticles,
did not show these features, albeit sharing a similar Au 4f XPS
spectrum. Still, differences between Au/NC-413 and Au/NC1073 can be distinguished in their normalized white line
intensity (0.73 vs. 1.15), a feature corresponding to d-electron
occupancy.[8] By comparing these values with those of gold
references, we can thus confirm that Au+ and Au3+ are the
major species in Au/NC-413 and Au/NC-1073, respectively.
The EXAFS analysis further reveals that Au was predominantly coordinated with Cl in Au/NC-413, and to N and/or O
in Au/NC-973 and Au/NC-1073 (Figure 2 e and Figure S6).
The best fitting of the first shell reveals a coordination
number (CN) of 2.0 for Cl, and 3.5 and 2.7, respectively, for N/
O in the two high-temperature samples (Figure S6 and
Table S4). This change in Au coordination atoms from Cl to
N/O upon high-temperature activation is fully supported by
surface elemental concentrations determined by XPS, as the
substantial amount of Cl in the low-temperature samples
(Cl:Au > 37, molar ratio) was almost completely removed
after treatment at T 973 K, but the N content remains rather
stable (Figure S7 and Table S3).
The remarkable stabilization of single Au atom on the NC
carrier was rationalized by density functional theory (DFT).
The most representative coordination environments were
determined (Figure 2 f) by analyzing the intrinsic stability
(Tables S6, S7 and Figure S8), the ability to bind Au or AuCl,
and the barriers for diffusion of gold species from the cavity to
the host surface of several nitrogen defects. The analysis of
formation energy (E) of an isolated AuCl molecule on
multiple cavities reveals that NC-Au-Cl moiety is thermodynamically stable on diverse structures. Since at low temperatures relatively high amounts of Cl are present, in Au/NC413 the photosensitive NC-Au-Cl species (from b1 to b6,
Figure 2 f) are likely displayed as Au0 (a1, E = ˇ1.48 eV) by
XPS analysis (vide supra). Upon increasing the temperature,
the removal of Cl in combination with the higher mobility of
Au atoms leads to the emergence of the Au+ feature at 573 K
corresponding to the oxidized-4-pyridinic structure (a4, E =
ˇ1.71 eV). The calculated Au 4f XPS shift is much higher
than that experimentally observed (1.74 vs. 0.80 eV), indicating likely Aud+ (1 < d < 3) species in a4. A further temperature increase involves higher diffusion of gold species that
enter the a6 cavity adopting a four-fold N-coordinated planar
geometry, which represents the global diffusion minima (E =
ˇ3.61 eV) and is observed as Au3+ by XPS and EXAFS.

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Figure 2. a) Aberration corrected HAADF-STEM micrographs of Au/NC-T and b) HAADF-STEM images of Au/AC-T catalysts, presenting the
comparative evolution of gold atoms on the two carriers. c) Au 4f core level XPS spectra and, d) ex situ Au L3 edge-normalized XANES spectra
together with e) the FT-EXAFS spectra of key gold catalysts and reference samples. f) Energy profiles for the surface diffusion of Au and AuCl
between 3-pyridinic, graphite, oxidized-4-pyridinic and 4-pyridinic, with the Bader charges and Au 4f XPS shifts (relative to an Au atom on the
graphite basal plane, Au yellow, C gray, N blue, O red, Cl green) presented in the bottom panel. The most representative gold species are
highlighted in bold.

The performance of the nanostructured gold catalysts
(Au/NC-T and Au/AC-T) was evaluated in the continuous
flow three-phase semi-hydrogenation of 1-hexyne and 2methyl-3-butyn-2-ol (Figure 3 a and Figure S9), two relevant
reactions in the production of a-olefins and synthesis of
intermediate for vitamins.[13] The single-atom catalyst Au/AC413 shows approximately three-fold higher activity than the
rest Au/AC samples containing mainly Au nanoparticles. On
the other hand, the two substrates display strikingly different
catalytic responses on the Au/NC-T series. While the 1hexyne hydrogenation activity is independent on the activation temperature and consequent Au speciation, a volcanolike curve is observed in the case of 2-methyl-3-butyn-2-ol. In

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accordance with the literature reports,[14] all these gold
systems give essentially full selectivity to the aimed alkenes.
In addition, HAADF-STEM examination on the two extreme
Au/NC-T catalysts confirmed that the atomic dispersion of
gold remains unaltered after the hydrogenation performance
(Figure S10).
To gain confidence on the above catalytic phenomenon on
Au/NC-T, we then extend the substrate scope (Figure 3 b and
Figure S9). The experimental results show that irrespective of
the chain length (1-dodecyne) and the position of the C⌘C
bond (4-octyne), the hydrogenation of unfunctionalized
substrates does not show any dependence on the gold
coordination sphere and speciation. Consistently, all adsorp-

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Figure 3. a) Comparative performance of Au/NC-T (circle) and Au/AC-T (square) catalysts in 1-hexyne and 2-methyl-3-butyn-2-ol semi-hydrogenation. b) Extrapolation of the substrate specificity to 1-dodecyne (light blue), 4-octyne (green), 3-methyl-1-pentyl-3-ol (red), and 4-pentyl-1ol (gray) over Au/NC-T. Reaction rate as a function of c) the Au oxidation state, based on the XPS fitting, and d) the nitrogen speciation.
Conditions: Wcat = 0.1 g, T = 303 K, P = 3 bar, FL(alkyne + toluene) = 1 cm3 minˇ1, and FG(H2) = 48 cm3 minˇ1. The alkyne conversion levels are
around 5–25 % under typical reaction conditions on all the catalysts evaluated.

tion energies of acetylene and 1-hexyne on relevant Au
species are within a narrow range (from ˇ0.18 to ˇ0.53 eV;
Figure 4 a), hinting that the molecules are bound in a similar
manner. As evidenced in Figure S11, the slight difference in
adsorption of the different substrates is due to the van der
Waals contributions arising from the different numbers of the
carbon atoms. Conversely, the reaction rate of 3-methyl-1pentyl-3-ol hydrogenation presents volcano behavior similar
to 2-methyl-3-butyn-2-ol, while the g-alcohol 4-pentyl-1-ol
behaves like the other unfunctionalized alkynes showing
structure insensitivity. Therefore, this molecular recognition
pattern[15] can be employed to identify the nature of the active
site in terms of Au oxidation state and cavity functionaliza-

tion. From the reactant adsorption heatmaps we have
identified that the most active site appearing at medium
temperatures (773–973 K) is a4, which contains the oxidized4-pyridine cavity and the Aud+ species. Neither Au+ nor Au3+
alone are activity descriptor for the hydrogenation, while
(oxidized) pyridinic N shows good correlation (Figures 3 c,d),
supporting the above conclusion. The Au@a4 ensemble
ensures that the adsorption of different alkynols (Figure 4 a)
is maximal for the a-ol due to the formation of a hydrogen
bond, which is by far the largest adsorption obtained among
all the alkynes studied (Figure S12). The reaction mechanism
was then simulated on this specific site and consists of
molecular hydrogen splitting by gold and the concomitant

Figure 4. a) Heatmap for the adsorption of acetylene (C2H2), 2-methyl-3-butyn-2-ol (a-alcohol, a-ol), 4-pentyl-1-ol (g-alcohol, g-ol) and 1-hexyne
(C6H10), and b) energy profile for the hydrogenation reaction of 2-methyl-3-butyn-2-ol on a single gold atom in the oxidized-4-pyridinic cavity.
c) Side view of DFT-optimized adsorption configuration of the intermediates for the reaction profile in (b).
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adsorption of 2-methyl-3-butyn-2-ol to the metal center and
the NˇO termination (Figure 4 b and Video S1). Then, the
two activated H are sequentially added to the triple bond,
forming the alkene that detaches from the material. To
rationalize the higher activity of Au single atoms over the
nanoparticle analogues, we have computed the full reaction
profile of 2-methyl-3-butyn-2-ol on the Au(211) surface
(Figure S13). By comparing the H2 activation step, the ratedetermining step for Au-based catalysts,[9b] between Au@4
and Au(211), it is evidenced that the activation is significantly
more thermodynamically favored on the single-atom sites
than on the Au surface (ˇ1.13 eV and ˇ0.17 eV, respectively).
Thus, the population of activated H is much higher on the
single-atom catalyst to further continue the reaction. Besides,
the a4 site occupied by gold is also stable against hydrogen
reduction (barrier 1.71 eV, Figure S14), that ensures the high
stability of the hydrogenation performance as evidenced by
HAADF-STEM of the used catalysts (see above).
In conclusion, we have developed an effective method to
entrap single Au atoms with N-doped carbon, and demonstrated how to finely tailor their nature (oxidation state and
coordination sphere) by taking advantage of their different
thermal stability in the cavities of the host. Assessment of the
diverse nanostructured gold catalysts in alkyne semi-hydrogenation confirms the higher activity of Au single atoms than
nanoparticles. Furthermore, the hydrogenation over the
single-atom Au/NC-T catalysts shows structure insensitivity
for unfunctionalized alkynes and alkynols with g-OH respect
to the triple bond, and structure sensitivity for alkynols with
a-OH. Theoretical methods link this unprecedented sensitivity behavior to the strong interactions between the adsorbed
alkynol molecules and the gold and oxidized-4-pyridinic N
ensemble, resulting in specific molecular recognition pattern
in alkyne semi-hydrogenation. Overall, the precise control of
the anchoring sites of single metal atoms developed in this
study represents a key step towards the rational catalyst
design, providing a novel way for breaking linear scaling
relations that can be extrapolated to other hydrogenations
and beyond.

Acknowledgements
This work was supported by the Swiss National Science
Foundation (project no. 200021-169679), ETH Research
Grant (ETH-40 17-1), and Spanish MINECO (CTQ201568770-R). Dr. Sharon Mitchell and Dr. Frank Krumeich, Dr.
´ ´
Roland Hauert, Bharath Tata, and Prof. Gordana Ciric´
Marjanovic are kindly acknowledged for performing the
microscopy, XPS, assistance with catalytic measurements, and
the fruitful discussions, respectively. ScopeM at ETH Zurich
is thanked for providing access to its facilities. E.F. thanks
MINECO La Caixa-Severo Ochoa for a predoctoral grant
through Severo Ochoa Excellence Accreditation 2014-2018
(SEV-2013-0319). BSC-RES is acknowledged for providing
generous computational resources.

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Conflict of interest
The authors declare no conflict of interest.
Keywords: alkyne semi-hydrogenation · gold ·
molecular recognition · N-doped carbon · single-atom catalysis
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Manuscript received: May 20, 2018
Accepted manuscript online: November 8, 2018
Version of record online: November 21, 2018

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