Evolution of catalytic activity driven by structural fusion of icosahedral gold cluster cores

Article abstract of DOI:10.1016/S1872-2067(20)63659-2

Atomically precise gold cluster catalysts have emerged as a new frontier in catalysis science, owing to their unexpected catalytic properties. In this work, we explore the evolution of the catalytic activity of clusters formed by the structural fusion of icosahedral Au₁₃ units, namely Au₂₅(SR)₁₈, Au₃₈(SR)₂₄, and Au₂₅(PPh₃)₁₀(SC₂H₄Ph)₅Cl₂, in the oxidation of pyrrolidine to γ-butyrolactam. We demonstrate that the structural fusion of icosahedral Au₁₃ units, forming vertex-fused (vf), face-fused (ff), and body-fused (bf) clusters, can induce a decrease in the catalytic activity in the following order: Au_bf > Au_ff > Au_vf. The structural fusion of icosahedral Au₁₃ units in the clusters does not distinguish the adsorption modes of pyrrolidine over the three clusters from each other, but modulates the chemical adsorption capacity and electronic properties of the three clusters, which is likely to be the key reason for the observed changes in catalytic reactivity. Our results are expected to be extendable to study and design atomically defined catalysts with elaborate structural patterns, in order to produce desired products.

Full text of DOI:10.1016/S1872-2067(20)63659-2

Chinese Journal of Catalysis 42 (2021) 245–250
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Communication
Evolution of catalytic activity driven by structural fusion of
icosahedral gold cluster cores
Dan Yang, Yan Zhu *
School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, Jiangsu, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Atomically precise gold cluster catalysts have emerged as a new frontier in catalysis science, owing
to their unexpected catalytic properties. In this work, we explore the evolution of the catalytic activ‐
ity of clusters formed by the structural fusion of icosahedral Au13 units, namely Au25(SR)18,
Au38(SR)24, and Au25(PPh3)10(SC2H4Ph)5Cl2, in the oxidation of pyrrolidine to γ‐butyrolactam. We
demonstrate that the structural fusion of icosahedral Au13 units, forming vertex‐fused (vf),
face‐fused (ff), and body‐fused (bf) clusters, can induce a decrease in the catalytic activity in the
following order: Aubf > Auff > Auvf. The structural fusion of icosahedral Au13 units in the clusters does
not distinguish the adsorption modes of pyrrolidine over the three clusters from each other, but
modulates the chemical adsorption capacity and electronic properties of the three clusters, which is
likely to be the key reason for the observed changes in catalytic reactivity. Our results are expected
to be extendable to study and design atomically defined catalysts with elaborate structural patterns,
in order to produce desired products.
Received 18 April 2020
Accepted 21 May 2020
Published 5 February 2021
Keywords:
Gold cluster
Structure fusion
Icosahedral unit
Oxidation of pyrrolidine
Active sites
Catalytic activity
© 2021, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
Recently, ligand‐capped Aun clusters (n = number of gold
atoms, ranging from a dozen to hundreds) with precise atomic
structures have attracted considerable interest. Significant
advances have been made in the synthesis and unambiguous
structural determination of atomically precise Aun clusters
[1–4]. These clusters are unique and substantially different
from their larger nanoparticle counterparts (typically 3–100
nm), as the former exhibit a range of fascinating optical, elec‐
tronic, and magnetic properties [5], which are not observed in
the latter. Small Aun clusters (relative to Au144, that is, smaller
than 1.7 nm) exhibit molecular‐like behavior such as sin‐
gle‐electron transitions or excitons; relatively larger ones (be‐
tween Au144 and Au333, i.e., 1.7–2.3 nm) exhibit intermediate
properties between molecular and metallic behavior [6–8];
clusters of size larger than that of Au333 (> 2.3 nm) can be clas‐
sified as metallic. Overall, the molecular or non‐metallic behav‐
ior of Aun clusters in excitonic and transition regimes is partic‐
ularly important in catalysis, where these systems often display
unexpected catalytic performances in some important chemical
reactions, and exhibit a clear correlation between properties
and structure[9–13].
More importantly, precisely controlling the structure of
clusters has proved to be a valuable strategy to improve cata‐
lytic activity and tune product selectivity. For example, alt‐
hough Au28(SPh‐tBu)20 and Au28(S‐c‐C6H11)20 have the same
Au20 core, the different surface structure of the two clusters can
result in different catalytic activity and selectivity toward CO2
hydrogenation [14]. The structural evolution from the
mono‐icosahedral [Au13(PPh3)10X2]3+ to the bi‐icosahedral
[Au25(PPh3)10(SC2H4Ph)5X2]2+ and even to the tri‐icosahedral
[Au37(PPh3)10(SC2H4Ph)10X2]+ cluster also illustrates the struc‐
ture‐dependent effects on CO oxidation [15].Even the replace‐
* Corresponding author. Tel: +86‐25‐89681696; E‐mail: zhuyan@nju.edu.cn
This work was supported by the National Natural Science Foundation of China (21773109, 91845104).
DOI: 10.1016/S1872‐2067(20)63659‐2 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 42, No. 2, February 2021

246
Dan Yang et al. / Chinese Journal of Catalysis 42(2021) 245–250
ment of only a central atom of a cluster by a foreign atom can
have substantial influence on the catalytic activity [16–18].
Therefore, the study of atomically precise clusters in catalytic
applications opens a new avenue for the design of catalysts
with improved efficiency, based on clusters with precise struc‐
tures. Herein, we explore the evolution of the catalytic activity
of clusters formed by the structural fusion of icosahedral Au13
units in different modes, namely, Au25(SR)18, Au38(SR)24, and
Au25(PPh3)10(SC2H4Ph)5Cl2, by evaluating their performance in
the oxidation of pyrrolidine to γ‐butyrolactam.
The compositions of the three clusters, as determined by
electrospray ionization mass spectrometry (ESI‐MS), were
Au25(SR)18, Au38(SR)24, and Au25(PPh3)10(SC2H4Ph)5Cl2 (Fig.
1(d)–(f)). The frameworks of the three clusters are shown in
Figs. 1(a)–(c). The Au25(PPh3)10(SC2H4Ph)5Cl2 cluster exhibits a
bi‐icosahedral structure obtained by the fusion of two icosahe‐
dral Au13 units joined by sharing one vertex gold atom (Fig.
1(a)), denoted as Auvf (vf: vertex‐fused). The Au38(SC2H4Ph)24 is
based on a face‐fused (ff) bi‐icosahedral Au23 core capped by a
second shell comprising the remaining 15 gold atoms. The two
icosahedra are fused together by sharing a common Au3 face,
yielding the Au23 core (Fig. 1(b)), denoted as Auff. The
Au25(SC2H4Ph)18 cluster features an icosahedral Au13 core en‐
capsulated by an external gold shell consisting of the remaining
12 gold atoms; in this case, body fusion (bf) results in the for‐
mation of an icosahedral Au13 core (Fig. 1(c)), named Aubf. As
we can see, the structural fusion of icosahedral Au13 units in‐
creases with the growth of the sharing Au atom.
The structural fusion of icosahedral Au13 units in the three
clusters results in changes in optical and electronic properties.
Fig. S1 shows that the UV‐vis optical bands of Aubf are observed
at 400, 447, and 683 nm. In the case of Auvf, the UV‐vis absorb‐
ance peaks appear at 320, 416, and 677 nm, whereas the ab‐
sorbance peaks of Auff are found at 1050, 750, 620, 520, and
490 nm. The UV spectra of the three clusters are in good
agreement with previously reported data [19–21]. Differential
potential voltammetry curves of the Auvf, Auff, and Aubf clusters
at room temperature are shown in Fig. S2. For the Auff clusters,
the first oxidation and reduction waves are found at +0.48 and
–0.71 V (vs. a quasi‐reference Ag electrode), respectively. This
reveals that the electrochemical energy gap of Auff is ~ 1.2 eV,
which is clearly different from that of Aubf (~ 1.63 eV) and Auvf
(~ 1.54 eV). After subtracting the charging energy, these values
were converted to actual HOMO‐LUMO gaps of 1.0, 1.34, and
Fig. 1. Structural fusion of icosahedral Au13 units: (a) vertex‐fused bi‐icosahedral Au25, (b) face‐fused bi‐icosahedral Au38, and (c) body‐fused icosahe‐
dral Au25. C and H atoms are omitted for clarity. Color labels: yellow, pink, and magenta: gold; light green: S; dark green: Cl; orange: P. ESI‐MS profiles
of (d) Auvf, (e) Auff,and (f) Aubf.

Dan Yang et al. / Chinese Journal of Catalysis 42 (2021) 245–250
247
1.21 V for Auff, Aubf, and Auvf, respectively [11,12,2]. These
measured gaps are close to the optical energy gaps of Auff (0.92
eV), Auvf (1.31 eV), and Aubf (1.18 eV), respectively, obtained
from the UV‐vis spectra by extrapolation to zero absorbance
(Fig. S1). The results show that the three gold clusters exhibit
non‐metallic behavior.
creased in the order Aubf > Auff > Auvf (Fig. 2(a)). The superior
activity of Aubf over Auvf and Auff was also reflected by the reac‐
tion rates of pyrrolidine over the three catalysts. The corre‐
sponding kinetic profiles for the reaction, shown in Fig. 2(b),
not only indicate that the pyrrolidine oxidation was approxi‐
mately a first‐order reaction, but also confirm that the reaction
rate constant of pyrrolidine oxidation over Aubf was approxi‐
mately 1.5–2 times higher than those of the Auvf and Auff reac‐
tion systems (29.84 × 103 h1 for Aubf, 14.79 ×103 h1 for Auvf,
and 17.21 ×103 h1 for Auff). The activation energy (Ea) for Aubf
An important question is whether the structural fusion of
icosahedral Au13 units can affect the catalytic activity and, if so,
how it can control the catalytic process. In order to address
these fundamental questions, the reaction of pyrrolidine with
O2 to form γ‐butyrolactam was selected to evaluate the catalytic
activity of the Aubf, Auff, and Auvf clusters. First, we examined
the catalytic performances of the three clusters loaded on
different supports (SiO2, CeO2, Al2O3, and P25) in the pyrroli‐
dine oxidation (Fig. S3). Among the different supports exam‐
ined, CeO2 was the best choice; in this case, upon the introduc‐
tion of O2 into the reaction system, the oxygen vacancies of
CeO2 can facilitate the oxidation reaction [22]. Note that CeO2
alone had limited activity (Table S1, entry 1), while the pure
ligands had no activity (Table S1, entries 3–4). These results
indicate that the catalytic performance was mainly determined
by the gold sites, rather than the ligands or supports.

1
was found to be 34.67 kJ·mol , which was lower than those of

1

1
other two catalysts (Auvf: 66.26 kJ·mol ; Auff: 52.71 kJ·mol )
(Fig. 2(c)). Notably, the influence of the different ligands of Aubf
(Fig. S4) on the pyrrolidine conversion, as shown in Fig. 2(d),
indicates that the pyrrolidine conversion varied with the chain
length of the ligands, with the highest value observed in the
case of the Aubf‐n‐butyl mercaptan, followed by
Aubf‐1‐octanethiol and then Aubf‐1‐dodecanethiol. The conver‐
sion was observed to increase with decreasing chain length,
which suggests that it is much easier for the pyrrolidine mole‐
cules to access the surface gold sites because of the smaller
steric hindrance compared to that of long‐chain thiolate mole‐
cules [23].
Among the three catalysts, Aubf/CeO2 exhibited the highest
activity (~ 80% conversion), while Auvf/CeO2 gave rise to the
lowest activity (~ 40% conversion). The catalytic activity de‐
Notably, the three catalysts only yielded trace amounts of
γ‐butyrolactam in air (0.4 MPa; Table S1, entries 5–7), while an
80
Aubf
a
0.0
Auvf
b
Auff
Auff
60
Auvf
Aubf
-0.4
-0.8
-1.2
40
20
0
0
10
20
30
40
50
0
10
20
30
40
50
Reaction time / h
Reaction time / h
80
d
-1.5
c
E
a
=
60
40
20
0
3
4
.
6
7
-2.0
-2.5
-3.0
-3.5
E
a
-
1
v
f
=
6
6
.
2
6
2.55
2.60
2.65
2.70
Aubf protected by different ligands
1000/T (K-1)
Fig. 2. (a) Catalytic oxidation of pyrrolidine over Auvf, Auff, and Aubf catalysts. Reaction conditions: catalyst (100 mg, 2 wt% Au), pyrrolidine (0.44
mmol), O2 (0.4 MPa), n‐butanol/H2O (4.5 mL/0.5 mL), 85 °C for different reaction times. (b) Linear dependence of ln(Ct/C0) on reaction time for the
oxidation of pyrrolidine over Auvf, Auff, and Aubf catalysts. (c) Arrhenius plot for pyrrolidine oxidation. (d) Conversion of pyrrolidine over Aubf catalysts
protected by different thiolate groups (n‐butyl mercaptan, 1‐octanethiol, and 1‐dodecanethiol) (48 h).

248
Dan Yang et al. / Chinese Journal of Catalysis 42(2021) 245–250
increasing yield of product was obtained in pressurized air (1
MPa; Table S1, entries 8–10). The reaction did not proceed
under N2 atmosphere (1 MPa; Table S1, entries 11–13), indi‐
cating that O2 was the oxidant in the reaction. Furthermore, we
studied the effect of H2O on the catalytic performances. As
shown in Fig. S5, the results show that a very low conversion of
pyrrolidine (< 10%) was obtained when H2O was absent. An
increase in the amount of H2O led to an increase in the conver‐
sion of pyrrolidine. When the H2O amount reached 0.75–1 mL,
the conversion decreased, due to ring‐opening hydrolysis of
γ‐butyrolactam. This suggests that water was necessary for this
oxidation reaction.
Under the optimized reaction conditions, we also extended
the substrate scope for the reaction. As shown in Table S2, five‐,
six‐, and seven‐membered cyclic amines could be oxidized to
produce the corresponding amides. In these reactions, Aubf still
showed superior activity to Auvf and Auff. Furthermore, the
clusters exhibited catalytic stability, up to a certain extent (Fig.
S6). The UV‐vis spectra of the spent clusters dissolved in CH2Cl2
indicate that the clusters showed no dramatic changes after the
catalytic reactions (Fig. S7). The transmission electron micros‐
copy (TEM) images show that the clusters exhibited no obvious
aggregation during the reactions (Fig. S8).
chains led to a higher hindrance, making it more difficult for CO
to access the surface Au sites. Therefore, these observations
were in line with the consequence of the catalytic activities,
supporting the observed evolution of the activity in the pyrrol‐
idine oxidation.
To obtain further insight into the reaction mechanism, the
adsorption of pyrrolidine onto the three clusters was investi‐
gated via FT‐IR spectroscopy. Fig. 4(a) shows that the peaks of
the –NH stretching vibration at 3325 cm of pyrrolidine ad‐
1
sorbed onto the clusters could hardly be seen. The comparison
with the IR spectra of pure pyrrolidine and of the three isolated
clusters (Fig. S9) reveals that the –NH group interacted with
the clusters, and the H atom of the –NH group was mostly re‐
moved from the amines and further activated. This indicates
that the adsorption of pyrrolidine on the three clusters in‐
volved its dehydrogenation process [9]. The X‐ray photoelec‐
tron spectroscopy (XPS) analysis of the three clusters further
highlights the changes in electronic structure upon the struc‐
tural fusion of Au13 units. As clearly shown in Fig. 4(b) and Ta‐
ble S3, the shift of the Au 4f binding energies of the Aubf cluster
was more positive than those of Auff and Auvf. The XPS data
show that the Au 4f binding energies of Aubf were positively
shifted, compared to those of Auff andAuvf. Au0 and Au+ species
were detected in the Au 4f spectra of the three nanoclusters. In
particular, the Au+ species appeared on the surface of
nanoclusters, mainly due to the electron donation from the
surface Au atoms to the ligands. The molar ratio of Au+ to Au0
was 0.7 for Aubf, 0.63 for Auff, and 0.51 for Auvf, revealing that
Aubf had a more positive charge in comparison with Auff and
Auvf. When O2 was used as an oxidant, it could directly attack
the Au atoms to form the Aucluster–O2(ad) species. With respect
to the NH activation of pyrrolidine, the presence of Au+ among
the surface Au atoms could substantially activate the nucleo‐
philic –NH group of pyrrolidine, since the surface Au+ species
are electrophilic [27]. Then, the activated –NH groups reacted
with the O2(ad) species on the surface of the Au clusters. Thus,
the more positive charge on the surface Au atoms can lead to a
higher catalytic ability to activate the nucleophilic –NH group.
On the basis of the XPS data, the activity decreased in the order
Fourier‐transform infrared (FT‐IR) spectra of CO adsorbed
onto the three catalysts were obtained to probe the nature of
the available Au sites. As shown in Fig. 3(a), the band at 2168
cm is assigned to gaseous CO and that at 2117 cm is at‐
tributed to CO molecules adsorbed onto the Auδ+ (0 < δ < 1)
sites [24–26]. As we can see, CO was only weakly adsorbed on
surface Au cations, but the relative intensities of CO adsorbed
on the three samples were different, and the chemical adsorp‐
tion capacity of CO on Au sites followed the order Aubf > Auff >
Auvf. It was proposed that the different chemical adsorption
capacities of CO on the clusters might account for the different
reactivity of the three catalysts. There were indications of a
higher number of catalytic active sites on the Aubf catalyst, fol‐
lowed by Auff and then Auvf. Fig. 3(b) shows that the peaks at
2158 and 2117 cm−1 decreased in intensity with increasing
chain length of the ligands, which reveals that the longer alkyl
1
1
Fig. 3. (a) In situ FT‐IR spectra of CO adsorbed onto the Auvf, Auff, and Aubf clusters; (b) In situ FT‐IR spectra of CO adsorbed onto the Aubf clusters
capped by different alkanethiol ligands.

Dan Yang et al. / Chinese Journal of Catalysis 42 (2021) 245–250
249
3325
a
Auvf
Auvf-pyrrolidine
Auff
Aubf
Auff-pyrrolidine
Aubf-pyrrolidine
Pyrrolidine
3500
3000
1500
1000
Wavenumber / cm-1
Fig. 4. (a) FT‐IR spectra of adsorbed species over the catalysts in the presence of pyrrolidine; (b) XPS profiles of Auvf, Auff, and Aubf clusters.
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Aubf > Auff > Auvf, which is essentially in agreement with the
experimental results.
̈
Overall, we proposed a possible mechanism for pyrrolidine
oxidation on gold clusters, based on the oxida‐
tion/hydration/oxidation sequence illustrated in Scheme 1. In
the first step, O2 is dissociated into O atoms on the gold cluster
sites and the β‐hydrogen is transferred to the gold sites with
release of H2O, to give the imine product. In the second and
third steps, the imine is hydrolyzed and then oxidized to pro‐
duce γ‐butyrolactam.
In summary, we reported the influence of the structural fu‐
sion of icosahedral Au13 units on the catalytic reactivity in the
oxidation of pyrrolidine to produce γ‐butyrolactam. The struc‐
tural fusion of icosahedral Au13 units in the clusters does not
distinguish the adsorption modes of pyrrolidine over the three
clusters from each other, but modulates the chemical adsorp‐
tion capacity and electronic properties of the clusters, which is
likely to be the reason for the observed changes in catalytic
reactivity. This work can be extended to explore the wider cat‐
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structural patterns to produce desired products.
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Scheme 1. Proposed mechanism of pyrrolidine oxidation on gold clus‐
ters.

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Dan Yang et al. / Chinese Journal of Catalysis 42(2021) 245–250
Graphical Abstract
Chin. J. Catal., 2021, 42: 245–250 doi: 10.1016/S1872‐2067(20)63659‐2
Evolution of catalytic activity driven by structural fusion of
icosahedral gold cluster cores
100
90
80
70
60
50
Dan Yang, Yan Zhu *
Nanjing University
Au
Body-fused
Atomically precise gold cluster catalysts have emerged as a new fron‐
tier in catalysis science and exhibited unexpected catalytic properties.
Here, we demonstrate that the structural fusion of icosahedral Au13
cores to form vertex‐fused (vf), face‐fused (ff), and body‐fused (bf)
structures can control the catalytic activity of the clusters.
Face-fused
Vertex-fused
Au
vf
Au
ff
Au
bf
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金团簇二十面体结构融合过程中其催化活性的演变
杨 丹, 祝 艳*
南京大学化学化工学院, 江苏南京210093
摘要: 近年来, 由有机配体保护的原子精确金属团簇在合成方面已取得了重要进展, 其独特的原子结构对一些化学反应产
生独特的催化效果. 原子精确的团簇催化剂明显不同于纳米颗粒催化剂和单原子催化剂, 是一种关联均相和多相的、原子
数目确定、尺寸均一、结构精确的新型催化剂. 从原子尺度上精确构筑团簇催化剂, 探究亚纳米尺度的微观结构对催化性
能的影响, 为常规催化剂所未能解决的关键科学问题提供解决的机会, 为在分子尺度上揭示催化作用机制以及准确关联催
化剂结构与催化性能提供新的研究体系, 具有重要的科学研究意义. 本文设计和使用了三种结构精确的金团簇催化剂, 即
Au25(PPh3)10(SC2H4Ph)5Cl2, Au38(SC2H4Ph)24和Au25(SC2H4Ph)18, 分别由二十面体结构的Au13单元通过中心顶点融合、面融
合、体相融合形成的(简写为Auvf、Auff和Aubf), 详细研究了这三个金团簇催化剂在二十面体Au13单元的结构融合过程中, 其
催化活性的演变规律. 在催化吡咯烷与O2反应制备γ-丁内酰胺反应中, 金团簇催化剂的催化活性顺序为Aubf > Auff > Auvf,
表明这三个金团簇中Au13单元的结构随着点、面、体的融合, 其催化活性随之增加. 同时研究发现, 对于同一个Au团簇催
化剂, 其表面硫醇配体的烷基链越短, 其催化活性越高, 这主要是由于短链硫醇分子的空间位阻较小, 吡咯烷分子更容易进
入催化剂的金表面, 接触到活性位点, 进行催化反应. 实验表明, 三个团簇金原子均带正电荷, 正价金物种可能是催化吡咯
烷与O2反应的催化活化物种. 研究发现, Aubf团簇表面的活性位数目高于Auff和Auvf团簇的, 因此Aubf的催化活性最高; 同
时, 团簇表面配体的烷基链越短, 其表面活性位数目也越多, 这也进一步解释了表面硫醇配体的烷基链越短, 其相应的金团
簇催化剂的催化活性越高的原因. 吡咯烷与O2在金团簇上反应的可能路径为O2在Au活性位上裂解的O原子和吡咯烷β-H
转移至Au活性位的β-H反应脱水后形成亚胺, 亚胺经过水解进一步氧化得到产物. 这项研究将为在原子层次上调变金属团
簇催化剂的结构进而改变其催化性能提供新的思路, 对精准设计和构筑高效催化剂具有一定的科学指导意义.
关键词: 金团簇; 结构融合; 二十面体结构; 吡咯烷氧化; 活性位点; 催化活性
收稿日期: 2020-04-18. 接受日期: 2020-05-21. 出版日期: 2021-02-05.
*通讯联系人. 电话: (025)89681696; 电子信箱: zhuyan@nju.edu.cn
基金来源: 国家自然科学基金(21773109, 91845104).
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).