Central Doping of a Foreign Atom into the Silver Cluster for Catalytic Conversion of CO2 toward C?C Bond Formation

Article abstract of DOI:10.1002/anie.201805319

Clusters with an exact number of atoms are of particular interest in catalysis. Their catalytic behaviors can be potentially altered with the addition or removal of a single atom. Now the effects of doping with a single foreign atom (Au, Pd, and Pt) into the core of an Ag cluster with 25 atoms on the catalytic properties are explored, where the foreign atom is protected by 24 Ag atoms (Au@Ag₂₄, Pd@Ag₂₄, and Pt@Ag₂₄). The central doping of a single atom into the Ag₂₅ cluster has a substantial influence on the catalytic performance in the carboxylation reaction of CO₂ with terminal alkyne through C?C bond formation to produce propiolic acid. These studies reveal that the catalytic properties of the cluster catalysts can be dramatically changed with the subtle alteration by a single atom away from the active sites.

Full text of DOI:10.1002/anie.201805319

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Accepted Article
Title: Central Doping of a Foreign Atom into the Silver Cluster for
Catalytic Conversion of CO2 toward C-C Bond Formation
Authors: Yuanyuan Liu, Xiaoqi Chai, Xiao Cai, Mingyang Chen,
Rongchao Jin, Weiping Ding, and Yan Zhu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201805319
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10.1002/anie.201805319
Angewandte Chemie International Edition
COMMUNICATION
Central Doping of a Foreign Atom into the Silver Cluster for
Catalytic Conversion of CO2 toward C-C Bond Formation
Yuanyuan Liu,[a] Xiaoqi Chai,[a] Xiao Cai,[a] Mingyang Chen,*[b] Rongchao Jin,[c] Weiping Ding,[a] and
Yan Zhu*[a]
affect the inherent catalytic properties of the original cluster.
In this work, we choose the Ag25 cluster as a model system
which can be viewed as a core-shell structure (Ag@Ag24), that is,
an Ag13 icosahedral core is encapsulated by an incompete shell
consisting of twelve Ag atoms in six extended staples (-S-Ag-S-
Ag-S-Ag-).[9] The Ag atom located at the icosahedral core of the
Ag@Ag24 cluster is replaced by a foreign atom (Au, Pd, and Pt)
to form Au@Ag24, Pd@Ag24, and Pt@Ag24 clusters, respectively,
which possess a foreign atom-centered core encapsulated by
24-silver atoms. It is anticipated that such single-atom doping
into the core of Ag@Ag24 clusters could modify the catalytic
nature of the original cluster and lead to completely different
catalytic behaviors. Here we report the catalytic conversion of
CO2 toward carbon-carbon bond formation using the centrally
single-atom doped clusters as catalysts and find a unique anion-
anion mechanism that promotes the carboxylation reaction.
Intriguingly, the atomic charge on the central atom governs the
catalytic properties of the doped clusters. The centrally doped
clusters exhibit a synergistic enhancement in the catalytic
efficiency, as compared to the original Ag@Ag24.
Abstract: Clusters with an exact number of atoms are of particular
research interest in catalysis. Their catalytic behaviors can be
potentially altered with the addition or removal of a single atom.
Herein we explore the effects of the single-foreign-atom (Au, Pd and
Pt) doping into the core of an Ag cluster with 25-atoms on the
catalytic properties, where the foreign atom is protected by 24 Ag
atoms (i.e., Au@Ag24, Pd@Ag24, and Pt@Ag24). The central doping
of a single atom into the Ag25 cluster is found to have a substantial
influence on the catalytic performance in the carboxylation reaction
of CO2 with terminal alkyne through C-C bond formation to produce
propiolic acid. Our studies reveal that the catalytic properties of the
cluster catalysts can be dramatically changed with the subtle
alteration by a single atom away from the active sites.
Atomically monodisperse clusters ideally composed of precise
numbers of atoms, e.g. ranging from a dozen to hundreds, have
attracted considerable attention because they exhibit specific
physicochemical properties that have so far not been observed
in the corresponding nanoparticles and bulk materials.[1] Small
metal clusters behave like molecules and exhibit strong quantum
confinement effects, and their unique behaviors are particularly
important for catalysis.[2] In addition, such clusters especially
permit a precise correlation of structures with catalytic properties
and also allow for identification of catalytically active sites on the
clusters.[3] Owing to the atomically precise synthesis developed
in the past decade, many ligand-protected clusters were
discovered.[4] These clusters often exhibit fascinating atom-
packing structures and catalytic properties. Contrary to the bulk
catalysts of which not all atoms are reactive, catalytic reactions
can occur on the surface atoms of a cluster catalyst, with the
Figure 1. (a) ESI-MS and (b) UV-vis spectra of Ag@Ag24, Au@Ag24, Pd@Ag24
and Pt@Au24 clusters capped by thiolates. Inset is the structural framework of
Au/Pd/Pt/Ag@Ag24. (SR = SPhMe2; SR moiety is omitted for clarity).
,
core atoms often directly involved in the reaction processes.[5 6]
In particular, every atom of the clusters may carry a potential
role in catalysis, and the catalytic performance thus can change
remarkably even with minor changes in the structure and
composition, such as the addition and removal of a single
atom.[7] It was reported that replacing the central gold atom of Au
clusters with a platinum atom caused drastic effects on the
catalytic activity.[8] However, it has not been unveiled to date
how a core atom of a cluster replaced with a foreign atom can
The four clusters capped by dimethylbenzenethiolate,
including Ag@Ag24, Au@Ag24, Pd@Ag24 and Pt@Ag24, were
synthesized through a kinetically controlled synthetic approach
and characterized by both mass spectrometry and UV-vis
absorption spectroscopy. The compositions of clusters were
determined by electrospray ionization mass spectrometry
(Figure 1a), confirming the atomic monodispersity and the exact
formula assigned to the as-prepared clusters. The doping
location of the foreign atoms in the Ag@Ag24 clusters (i.e., at the
center, in the icosahedral shell, or in one of the dimer staples)
was manifested in the UV-vis spectra (Figure 1b), which are well
in accordance with the previously reported spectra for the
centrally doped Ag25 clusters.[10] The schematic structure of the
centrally single-atom doped clusters was shown in the inset of
Figure 1b (noted that the thiolate is omitted for clarity).
[a]
Y. Liu, X. Chai, X. Cai, Prof. W. Ding, Prof. Y. Zhu
Key Lab of Mesoscopic Chemistry, School of Chemistry and
Chemical Engineering, Nanjing University
No. 163 Xianlin Road, Nanjing 210093, P.R. China
E-mail: zhuyan@nju.edu.cn
[b]
[c]
Dr. M. Chen
Beijing Computational Research Center, Beijing 100193, P.R. China
E-mail: mychen@csrc.ac.cn
Prof. R. Jin
The carboxylation of CO2 with terminal alkyne into propiolic
acid was selected as a model reaction to explore the influence of
the single-foreign-atom central doping of Ag@Ag24 cluster on the
catalytic properties. As shown in Figure 2, the propiolic acid
derivatives were synthesized from CO2 and terminal alkynes
Department of Chemistry, Carnegie Mellon University
Pittsburgh, Pennsylvania 15213, United States
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201805319
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using Au@Ag24, Pd@Ag24, Pt@Ag24 and Ag@Ag24 clusters,
respectively, as catalysts. Among the four catalysts, Au@Ag24
catalyst gave rise to the highest product yield (> 92%), while
Ag@Ag24 catalyst exhibited the lowest activity. It is noted that
Pd@Ag24 and Pt@Ag24 clusters had no notable difference in
their catalytic performances of the carnoxylation of CO2 with
terminal alkyne. In detail, the catalytic activity can be presented
measure the chemical adsorption capacity of CO2 onto these
clusters. As shown in Figure 3c, the principal desorption band of
the chemisorbed CO2 on Au@Ag24 was centered around 328 oC,
which is lower than other three clusters. Pd@Ag24 and Pt@Ag24
clusters had a desorption band of adsorbed CO2 at 333 and 352
oC, respectively. For Ag@Ag24, most CO2 was desorbed at 352
oC. Moreover, the amounts of adsorbed CO2 on the four clusters
were deduced by integrating the peak areas. The chemical
adsorption capacity of CO2 over the clusters decreased in the
following order: Au@Ag24 > Pd@Ag24 > Pt@Ag24 > Ag@Ag24. It
is proposed that the distinct chemical adsorption capacity of CO2
on the clusters might partially account for the different
reactivities of the four catalysts.
in a descending sequence: Au@Ag24 > Pd@Ag24 ～ Pt@Ag24
>
Ag@Ag24. Notably, the investigation of the catalytic properties of
recycled catalysts illustrates that the doped catalysts can afford
a catalytic stability to a certain extent, compared to Ag@Ag24
catalyst. As shown in Figure S1, the doped catalysts had a slight
decrease in activity with multiple cycles, whereas the activity of
the original Ag@Ag24 catalyst suffered an obvious drop. The UV-
vis analysis of the spent catalysts (Figure S2) indicates that the
doped clusters appeared to undergo no dramatic change after
cycles. The catalytic results unequivocally suggest that the
central doping of Au, Pd and Pt into Ag25 can bring about a
significant increase in the essential reactivity and stability.
Figure 3. FT-IR spectra of the adsorbed species on the four clusters in the
presence of phenylacetylene: (a) without Cs2CO3 and (b) with Cs2CO3. (c)
CO2-TPD profiles of the four clusters. (d) FTIR spectra of CO adsorbed onto
the four clusters.
Figure 2. The catalytic carboxylation of terminal alkynes with CO2 over
Au@Ag24, Pd@Ag24, Pt@Ag24 and Ag@Ag24 clusters, respectively. Reaction
conditions: catalyst (10 mg), alkyne (1.0 mmol), Cs2CO3 (1.2 mmol), CO2 (1
MPa), 50 oC, 12 h.
The electron densities of the doped clusters were analyzed
by FT-IR of CO adsorbed on the clusters, because the stretching
frequency of CO depends strongly on the electronic state of the
adsorption sites. Figure 3d displays the spectra of CO adsorbed
onto the clusters. The initial spectrum of Ag@Ag24 cluster
showed a band centered at 2041 cm-1, which is assigned to CO
linearly bonded to metallic silver sites.[13] CO molecules
We now turn to gain deep insight into the fundamental origin
of catalytic reactivity of Ag clusters boosted by the central
doping of the foreign atom. The adsorption of terminal alkyne
such as phenylacetylene on the four clusters was detected by
Fourier transform infrared spectroscopy (FT-IR). When Cs2CO3
was absent from the reaction system, as shown from Figure 3a,
we were able to distinguish the two dominate stretching peaks of
alkyne: ≡ C-H stretching vibration at 3290 cm-1 and C≡ C
stretching vibration at 2105 cm-1, indicating that the whole C≡C
bond of the alkyne interacts with the clusters.[11] For comparison,
when Cs2CO3 was added into the reaction system, an interesting
phenomenon was observed: the peak at 3290 cm-1 disappeared
and the peak at 2105 cm-1 shifted to 2145 cm-1 (Figure 3b). It
reveals that the H atom of ≡C-H group is removed from the
alkyne and the terminal of C≡C bond of alkyne interacts with
clusters and becomes activated.[12] The existence of Cs2CO3 is
thus crucial for the adsorption and activation of the terminal of C
≡C bond of alkyne on the clusters. It should be pointed out that
if there was no Cs2CO3 in the reaction, the product yield was
very low (less than 10%) for the four catalysts. In our catalytic
system, the carboxylation of CO2 with terminal alkyne catalyzed
by the four clusters should proceed via the deprotonation-
induced activation pathway.
adsorbed on the doped clusters exhibited
a blue-shifted
stretching at a higher frequency, i.e. 2066, 2053 and 2058 cm-1
for Au@Ag24, Pd@Ag24 and Pt@Ag24, respectively. This
definitely indicates that the electron density on the silver atoms
decreased, namely the doped clusters possessed lower electron
densities, via the elelctron donation from the external silver
atoms to the central dopants.
X-ray photoelectron spectroscopy (XPS) of the four clusters
further demonstrates the alteration of the electronic structure of
the centrally doped clusters with respect to that of Ag@Ag24
cluster. As shown in Figure 4a, Ag@Ag24 exhibited Ag 3d 3/2
and 5/2 peaks at 374.7 and 368.6 eV, respectively. In contrast,
the Ag peaks for other three doped clusters appeared at lower
binding energies. This reflects the fact that the silver clusters
underwent the electronic perturbation when the core atom of
Ag@Ag24 was replaced by a dopant atom, and concretely, the
residual 24-atom Ag of the doped clusters charged more
positively via electron donation from Ag to the dopant, which
results in a decrease in electron density of the exterior silver
atoms.[14] Moreover, the XPS data of the dopants were also
In addition, the desorption of CO2 as one of reaction
molecules on the four clusters is also responsible for the distinct
catalytic performance among the catalysts, therefore, the CO2
temperature-programmed desorption (TPD) was conducted to
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10.1002/anie.201805319
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studied (Figure 4b-d). Regardless of the dopant being Au, Pd or
Pt, a similar trend was found, that is, Au 4f, Pd 3d and Pt 4f
binding energies were positive core level shifts with respect to
those of bulk metals, which is tentatively attributed to the
reduced core-hole screening in subnanometer metal
clusters.[3a,15-17] These findings further confirm the existence of
strong interactions between the dopant and the external silver
atoms, leading to different electronic states between the doped
clusters and the original cluster.
Ag (-1.7) according to the natural atomic charge analysis (Figure
5b). For Ag@Ag24, the core Ag is a relatively weak anion, so the
attraction between core Ag and outer-Ag and the repulsion
between the core Ag and the α-C are also weak; therefore, the
attraction between the outer-Ag and the α-C can tear the out-
Ag‒S bond apart. When the core M is heavily negatively
charged as found in Pd@Ag24 and Pt@Ag24, it can repel the α-C
and cause the geometry deformation. When the negative charge
on core M is median as found in Au@Ag24, a balance of the
electrostatic interactions is reached so that the cluster exhibits
high stability and the [CH3C≡C]- has high activity (due to the
weaker outer-Ag‒α-C interaction). The superior stability of
Au@Ag24 among the four catalysts under the influence of strong
base is also indicated by the natural atomic charge spectra.
Upon the adsorption of [CH3C≡C]-, the natural atomic charge
band for the inner shell Ag12 splits for Ag@Ag24, Pd@Ag24 and
Pt@Ag24 but remains essentially the same for Au@Ag24,
indicating more substantial changes in the electronic structures
of Ag@Ag24, Pd@Ag24 and Pt@Ag24.
Figure 4. (a) Ag 3d, (b) Au 4f, (c) Pd 3d, and (d) Pt 4f XPS profiles of the
clusters.
To
obtain
molecular-level
understanding
into
the
enhancement of carboxylation of CO2 with terminal alkyne by the
core-doped clusters, density functional theory calculations (DFT)
were performed. When promoted by Cs2CO3, the terminal alkyne,
exemplified by CH3C≡CH, loses a proton to form its conjugate
base [CH3C≡C]-. The [CH3C≡C]- can either recombine with the
Cs+ and [PPh4]+ in the solution ([PPh4]+ is the cation to neutralize
[Ag24M(SR)18]x- anions in the clusters), or adsorb onto the doped
[Ag24M(SR)18]x- (M@Ag24). The DFT results show that despite
M@Ag24 being anion clusters, the adsorption of [CH3C≡C]- can
occur at the outer-Ag (Figure 5a; the M@Ag24 contains a core M,
an inner Ag12 shell and an outer Ag12 shell) under favorable
conditions. At 0 K, the adsorption of [CH3C≡C]- is thermally
netural on Au@Ag24, endothermic by ~35 kcal/mol on Pd@Ag24
and Pt@Ag24, and exothemric by 11 kcal/mol on Ag@Ag24. The
weak adsorption of [CH3C≡C]- on the M@Ag24 is due to the
radial charge distribution of M@Ag24 where the core metal is
negatively charged, the inner Ag12 shell is netural, and the outer
Ag12 shell is positive (Figure 5b). Therefore, the adsorption
structure can be described by a simpifistic core M‒dipole model,
where the dipole is formed by the negative α-C of [CH3C≡C]- and
the positive outer-Ag. The geomtries of [CH3C≡C]-[M@Ag24]x-
are different (Figure 5a). In [CH3C≡C]-[Ag@Ag24]-, one of the two
Ag-S bonds between the adsorbent outer-Ag and thiol is broken.
In [CH3C≡C]-[Pd@Ag24]2- and [CH3C≡C]-[Pt@Ag24]2-, the
adsorbent outer-Ag atom is dislocated outwards. In [CH3C≡C]-
[Au@Ag24]-, no significant geometry change is found for the
cluster. The differences in the geometry changes upon the
adsorption of [CH3C≡C]- suggest the stabilities of the M@Ag24 in
Lewis basic solution are: Au@Ag24 (unchanged) > Pd@Ag24 and
Pt@Ag24 (distorted) > Ag@Ag24 (collapsed). The outer-Ag‒α-C
bond distances that reflect the bond strength of the outer-Ag‒α-
C bonds are 2.03 Å for Ag@Ag24 < 2.06 Å for Pd@Ag24 and
Pt@Ag24 < 2.18 Å for Au@Ag24, in a different order than that of
the adsorption energies. This is because the adsorption energy
is not only contributed by the outer-Ag‒α-C attraction, but also
contributed by the electrostatic interactions between the core M
and the dipole. The latter differs for the four clusters, as the
charge negativities of the core M are Pd/Pt (-2.5) > Au (-1.9) >
Figure 5. (a) DFT-optimized geometries for the adsorption products of
[CH3C≡C]- on M@Ag24 (M = Au, Pd, Pt, Ag). (b) Natural atomic charge spectra
for the adsorption of [CH3C≡C]- on M@Ag24. Purple lines indicate the natural
atomic charges for M@Ag24. ‘c’ denotes core metal M, ‘i’ denotes the inner
Ag12 shell, ‘o’ denotes the outer Ag12 shell, ‘m’ denotes the methyl C, ‘s’
denotes S, ‘h’ denotes H, and ‘α’, ‘β’, and ‘γ’ denote α-, β- and γ- C of
[CH3C≡C]-. (c) Potential energy surface (at DFT level in kcal/mol) for the the
[CH3C≡C]-
[Ag24M(SR)18
+
]
CO2
→
[CH3C≡C-COO]- reactions in the solutions of the
catalysts. The atoms are colorized as the following: C: brown,
x-
H: white, O: red, S: yellow, P: black, Cs: purple, core Ag: dark blue, core Au:
orange, core Pd: dark green, core Pt: light green, inner-Ag: cyan, and outer-Ag:
pink.
The potential energy surface for the reactions of [CH3C≡C]-
and CO2 to form [CH3C≡C-COO]- (propiolate) in the solutions of
the M@Ag24 is shown in Figure 5c. In contrast to the weak
anion-anion adsorption of [CH3C≡C]- on M@Ag24, the
recombination of [CH3C≡C]- and the Cs+ and [PPh4]+ cations in
the soultions are extremely exothemic. The cations in the
solutions serve as [CH3C≡C]- traps and are detrimental to the
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COMMUNICATION
R. Parida, O. F. Mohammed, O. M. Bakr, Angew. Chem. Int. Ed. 2016, 55,
922-926.
CO2 carboxylation reaction. In contrast, the M@Ag24 catalysts
serve as the temporary hosts for the active [CH3C≡C]- species,
and facilitate the formation of [CH3C≡C-COO]-. In addition, due
to the excessively strong bonding of [CH3C≡C]- and [CH3C≡C-
COO]- to the cations, the dissociations of the Cs and PPh4
propiolates are highly endothermic, which inhibits the formation
of the propiolic acid. Overall, the M@Ag24 catalysts show unique
mechanism in catalyzing the coupling reactions in the presence
of bases in the solution.
[11] A. Arcadi, Chem. Rev. 2008, 108, 3266-3325.
[12] H. Yamamoto, P. Maity, R. Takahata, S. Yamazoe, K. Koyasu, W.
Kurashige, Y. Negishi, T. Tsukuda, J. Phys. Chem. C 2017, 121,
10936−10941.
[13] J. Muslehiddinoglu, M. A. Vannice, J. Catal. 2003, 213, 305-320.
[14] I. L. Salido, D. C. Lim, Y. D. Kim, Surf. Sci. 2005, 588, 6-18.
[15] A. K. Santra, D. W. Goodman, J. Phys. Condens. Matter 2003, 15, 31-62.
[16] J. Eadnik, A. Benhmid, V. N. Kalevaru, M. M. Pohl, A. Martin, B. Lucke, U.
Dingerdissen, Angew. Chem. Int. Ed. 2005, 44, 6771-6774.
[17] S. Xie, N. Lu, Z. Xie, J. Wang, M. J. Kim, Y. Xia, Angew. Chem. Int. Ed.
2012, 51, 10266-10270.
In summary, we report for the first time the influence of the
central doping of a foreign atom into Ag25 clusters on the
catalytic carboxylation of CO2 with terminal alkyne through C-C
bond formation to produce propiolic acid. The foreign core atom
of clusters indeed contributes to the catalytic properties, even
though the foreign atom is surrounded by the external atoms
and away from active sites. Our results provide fundamental
understanding of how to tune the catalysis of clusters by
removal and addition of a single atom of a cluster and also shed
light on the strategy for controlling the doping position to tune
the catalytic properties.
Acknowledgements
Y.Z. acknowledges financial support from Nanjing University,
Fundamental Research Funds for the Central Universities,
and National Natural Science Fundation of China (21773109).
M.C. acknowledges funding from National Natural Science
Foundation of China (U1530401).
Keywords: central doping • core atom • cluster • catalysis • CO2
conversion
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
Au@Ag24
Pd@Ag24
Pt@Ag24
Ag@Ag24
The catalytic properties of the cluster
catalysts can be dramatically changed
with the subtle alteration by a single
atom away from the active sites.
Details were revealed that the central
doping of a single foreign atom into
the Ag25 cluster has a substantial
influence on the catalytic performance
in the carboxylation reaction of CO2
with terminal alkyne through C-C bond
formation to produce propiolic acid.
Author(s), Corresponding Author(s)*
catalyst
CO
2
+
100
80
Page No. – Page No.
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Title
40
20
((Insert TOC Graphic here)
0
de
ehy
lene
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in
ld
enza
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ene
ene
lu
lu
thynylto thynylto
henylac
thynylanil
orop
thynylb
Phenylac
4-E
hl
4-C
4-E
3-E
4-E
Layout 2:
COMMUNICATION
Author(s), Corresponding Author(s)*
Page No. – Page No.
Title
Text for Table of Contents
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