Maximizing the Number of Interfacial Sites in Single-Atom Catalysts for the Highly Selective, Solvent-Free Oxidation of Primary Alcohols

Article abstract of DOI:10.1002/anie.201803272

The solvent-free selective oxidation of alcohols to aldehydes with molecular oxygen is highly attractive yet challenging. Interfacial sites between a metal and an oxide support are crucial in determining the activity and selectivity of such heterogeneous catalysts. Herein, we demonstrate that the use of supported single-atom catalysts (SACs) leads to high activity and selectivity in this reaction. The significantly increased number of interfacial sites, resulting from the presence of individually dispersed metal atoms on the support, renders SACs one or two orders of magnitude more active than the corresponding nanoparticle (NP) catalysts. Lattice oxygen atoms activated at interfacial sites were found to be more selective than O₂ activated on metal NPs in oxidizing the alcohol substrate. This work demonstrates for the first time that the number of interfacial sites is maximized in SACs, providing a new avenue for improving catalytic performance by developing appropriate SACs for alcohol oxidation and other reactions occurring at metal–support interfacial sites.

Full text of DOI:10.1002/anie.201803272

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Angewandte
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Accepted Article
Title: Maximizing interfacial sites by single-atom catalyst for solvent-
free selective oxidation of primary alcohol
Authors: Botao Qiao, Tianbo Li, Fei Liu, Yan Tang, Lin Li, Shu Miao,
Yang Su, Junying Zhang, Jiahui Huang, Hui Sun, Masatake
Haruta, Aiqin Wang, Jun Li, and Tao Zhang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201803272
Angew. Chem. 10.1002/ange.201803272
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Angewandte Chemie International Edition
10.1002/anie.201803272
COMMUNICATION
Maximizing interfacial sites by single-atom catalyst for solvent-
free selective oxidation of primary alcohol
#
#
#
Tianbo Li, Fei Liu, Yan Tang, Lin Li, Shu Miao, Yang Su, Junying Zhang, Jiahui Huang, Hui Sun,
Masatake Haruta, Aiqin Wang, Botao Qiao,* Jun Li,* Tao Zhang
Dedication
Abstract: Solvent-free selective oxidation of alcohol by molecular
oxygen to aldehyde is highly attractive yet challenging. Interfacial
sites between metal and oxide support is crucial in determining the
activity and selectivity. Here we demonstrate that supported single-
atom catalysts (SACs) are highly active and selective for this
reaction. The much increased interfacial sites resulted from singly
dispersed metal atoms onto the support make SACs one or two
orders of magnitude more active than the nanoparticle (NP) catalyst.
The lattice oxygen activated at the interfacial sites is found to be
simultaneously achieving high activity and high selectivity is
[2]
rather difficult.
Currently,
developed for selective oxidation of alcohols, in both gas and
a
series of heterogeneous catalysts are
[
3]
liquid oxidation process. The nature of the active sites is,
however, still not fully understood. A recent study, however, has
[
4]
revealed that the interfacial sites served as the active sites.
The understanding of the critical role of the interfacial sites for
the alcohol oxidation helps rational design of selective oxidation
catalysts with improved performance by engineering interfacial
sites of metal/oxide. Interfacial sites are also found to be critical
2
more selective than O activated on the metal NPs. This work for the
first time demonstrates that SACs can maximize the interfacial sites,
thus providing a new avenue to improve catalytic performance by
developing appropriate SACs for alcohol oxidation and other
reactions occurred at the metal-support interfacial sites.
[
5]
for a variety of reactions, although the current practice of
assembling metal NPs and oxide NPs usually introduces rather
[
5a]
limited interfacial sites.
Recently, single-atom catalysis is quickly emerging as a
[6]
new frontier in the field of heterogeneous catalysis. Different
from nanocatalysts, SACs maximize not only the atom efficiency
of supported metals but also the interfacial sites, in contrast to
NPs where the unexposed inner atoms cannot contact the
support to form interfacial sites. Due to the uniform active site,
SACs tend to have superior catalytic performance in selective
oxidation of primary alcohol in comparison with their NP
Aldehydes are versatile building blocks and intermediates widely
used in fine chemical synthesis due to their ability to easily react
with nucleophiles. Nowadays, stoichiometric oxidations of
alcohols by using expensive and toxic inorganic oxidants to
obtain aldehydes are still commonly used. These processes,
however, suffer from the formation of by-products and wastes,
which leads to low atom economy and environmental issues.
Greener and more economical alternative processes are thus
highly desired. Selective oxidation of alcohols with molecular
oxygen by heterogeneous catalysis process, especially in the
solvent-free manner, is quite attractive but challenging for
[
1]
[4]
counterparts.
In this communication, we have shown that SACs can
create maximal interfacial sites as exemplified by ceria
supported SACs (M
counterparts. CeO
alcohol oxidation with optimizing preparation and can provide
more vacancies to anchor a high loading amount of noble metal
1
/CeO
2
, M = Au, Pt) compared with their NP
2
is used here as it is a superb support for
[
7]
[
+]
[+]
T. Li, Dr. F Liu, Dr. L Li, Prof. S. Miao, Y. Su, Dr. J. Zhang, Prof.
J. Huang, H. Sun, Prof. M. Haruta, Prof. A. Wang, Prof. B. Qiao,*
Prof. T. Zhang
[
8]
to form stable SACs due to quantum primogenic effect.
Au /CeO is found to be highly active, selective, and stable for
1
2
Dalian Institute of Chemical Physics
Chinese Academy of Sciences
Dalian 116023 (China)
selective oxidation of benzyl alcohol to benzaldehyde without
any solvent and the turnover frequency (TOF) reaches as high
-1
E-mail: bqiao@dicp.ac.cn
as 3,036 h , about one order of magnitude higher than the
standard Au NP catalyst. More importantly, Au /CeO shows
much higher selectivity to benzaldehyde than Au/CeO NP
junli@tsinghua.edu.cn
1
2
[
+]
Y. Tang, Prof. J. Li*
2
Department of Chemistry and Key Laboratory of Organic
Optoelectronics & Molecular Engineering of the Ministry of
Education,
Tsinghua University,
Beijing 100084, China
catalyst and excellent reusability in recycle test. This catalytic
process can also be operated in a fix-bed model running
smoothly with a high conversion and selectivity for at least 500
hours without any deactivation. This distinctive performance of
T. Li
1 2
Au /CeO catalyst can be extended to other metals (e.g., Pt),
University of Chinese Academy of Sciences
Beijing 100049, China
Dr. J. Zhang, Prof. J. Huang, Prof. M. Haruta
Gold Catalysis Research Center
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
Dalian 116023, China
thus providing a general strategy of engineering the interfacial
sites for catalyst development of solvent-free selective oxidation
of alcohols.
The CeO
2
support was homemade by a co-precipitation
2
method. CeO supported single metal catalysts were prepared
[
8a]
Prof. M. Haruta
Research Center for Gold Chemistry and Department of Applied
Chemistry, Graduate School of Urban Environmental Sciences
Tokyo Metropolitan University
by a feasible adsorption method with a metal loading of ~0.04
[8a, 8b]
wt% and denoted as M
additional studies were performed with 0.98 wt% Au/CeO
provided by Haruta Gold Inc. (denoted as Au/CeO -RRCe) and
% Pt/CeO -NP prepared by depositing the colloidal Pt NPs with
1
/CeO
2
(M = Au, Pt).
For comparison,
2
Tokyo 192-0397, Japan.
[
+] these authors contribute equally to this work.
2
1
2
Supporting information for this article is given via a link at the end of
the document.
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Angewandte Chemie International Edition
10.1002/anie.201803272
COMMUNICATION
average size of 6.5 nm. The detailed procedures for all sample
preparation, characterization and test were presented in the
Support Information (SI).
Representative aberration-corrected high-angle annual
dark-field scan transmission electron microscopy (HAADF-
1 2
presented in Table S2. It turns out that Au /CeO is highly active
and selective for a broad range of primary alcohols: Almost all
the selectivities to corresponding aldehydes are higher than 90%
-1
and the TOFs vary from 3 000 to ~17 000 h .
STEM) images of Au
magnification no nanoclusters (NCs) or NPs were observed
Figure S1A) while with high magnification many Au atoms were
clearly observed (in circles, Figure 1a), suggesting only isolated
Au atoms exist. For Au/CeO -RRCe the Au NPs were clearly
1
/CeO
2
revealed that with relatively low
Table 1 Catalytic performance of various gold supported ceria for benzyl
a
alcohol oxidation
O
(
OH 0.04%-Au1/CeO2
H
2
Mole ratio
PO
2
Conv.
(%)
Sel.
(%)
TOF
observed and the average size is about 4.5 nm (Figure S2),
which is consistent with the value provided by the supplier.
Entry
Catalyst
of Au/M
-
1
(
MPa)
(h )
(ppm)
b
1
CeO
2
/
0.5
0.5
ND
/
/
a
a
b
2
1
Au /CeO
2
7
15.9
98
3036
Au/CeO
RRCe
2
-
a,c
3
155
0.5
0.5
37.0
72
288(1309)
d
4
1
Au /CeO
2
2
7
7
51.9
9.5
93
98
3283
1803
5
1
Au /CeO
0.15
Au/CeO
RRCe
2
-
e
6
--
0.5
30.3
15.0
4.8
94
90
87
--
f
7
1
Pt /CeO
2
8
0.5
0.5
3043
48(739)
Pt/CeO
2
-
c,f
8
156
NP
Figure 1 Typical aberration-corrected HAADF-STEM images of 0.04 wt%
Au /CeO catalyst before (a) and after 5-cycle reactions (b)
a
Reaction conditions: benzyl alcohol, 5 mL; 150 mg catalyst, reaction time, 8
1
2
o
b
c
2
h; temperature, 150 C. CeO support was used. TOF in parenthese is
d
e
f
normolized with surface atoms. Reaction time, 24 h. the pressure of N
2
o
The alcohol oxidation was performed at 150 C for 8 hours
o
Reaction time, 6 h; temperature, 120 C.
in a solvent-free manner (details are presented in SI). Benzyl
alcohol was primarily tested in this work as it is widely used as
The adsorption behaviors of benzyl alcohol and
benzaldehyde were further studied. The adsorption heats of
benzyl alcohol on both samples are weak and similar (7 - 8 KJ
mol , Figure S3A), thus the catalytic performance is mainly
determined by the adsorption behavior of benzaldehyde. Our
[
2b, 4, 9]
typical probe molecule for primary alcohol oxidation.
support was first tested as a control, showing that CeO
CeO
itself is
2
2
inactive at all in this condition (Table 1). After introducing small
amount of Au atoms (0.04 wt%), the sample became highly
active and yielded a 15.9% conversion during 8-hour reaction
with an Au/substrate mole ratio of only 7 ppm, which is
-1
measurements (Figure S3B) show that the adsorption heat of
-1
benzaldehyde on Au
lower than that on Au/CeO
1
/CeO
2
is ~19.8 kJ mol , which is much
-
1
equivalent to a TOF of ~3000 h (Table 1 entry 2). Notably, the
selectivity to benzaldehyde is as high as 98%.
-1
2
-RRCe (~40.5 kJ mol ). Therefore,
1 2
the weaker adsorption of benzaldehyde on Au /CeO sample is
likely to partially contribute to the high selectivity due to its easy
desorption to avoid over-oxidation.
For comparison, commercially available 0.98 wt%
Au/CeO -RRCe was also tested with identical condition. To our
2
surprise, despite using about 20 times Au amount (Au/substrate
ratio of ~150 ppm), only double benzyl alcohol conversions were
It is believed that for selective oxidation the lattice oxygen
of a reducible metal oxide might serve as a versatile and more
-1
obtained, yielding a TOF of 288 h (entry 3). The TOF value is
at the same level compared with the previous reported oxide
supported Au NP catalysts (Table S1), but it is at least one order
[10]
selective oxidizing agent than gaseous dioxygen, despite lack
of solid evidence. To verify this, we performed a benzyl alcohol
oxidation under the N
RRCe catalyst (details are presented in SI) and found that with a
similar conversion the selectivity on Au/CeO -RRCe was
significantly improved from ~72% to ~94% (Table 1, entry 6),
which is close to that on the Au /CeO . This result clearly
2 2 2
atmosphere without gas O on Au/CeO -
1 2
of magnitude lower than the Au /CeO sample. Moreover, the
selectivity to benzaldehyde with this sample was much lower as
well, yielding over-oxidation production (benzoic acid) as the
major byproduct.
2
1
2
To exclude the possibility that the higher selectivity on
corroborates the critical role of lattice oxygen in determining the
high selectivity. However, it should be noted that the selectivity
Au
extended the reaction time to 24 hours on Au
much higher conversion (51.9%), yet the selectivity (93%)
remains much higher than those on Au/CeO -RRCe sample,
entry 4. Even upon significantly decreasing the O pressure to
near ambient pressure (0.15 MPa), the TOF on Au /CeO is still
as high as ~1800 h , implying that the activity has little
dependent on O concentration. Other alcohols were further
tested to explore the scope of substrates and the results were
1
/CeO
2
results from the lower benzyl alcohol conversion, we
1
/CeO and found a
2
(
94%) obtained at a conversion of ~30% as a result of lattice
oxygen oxidation is just similar to that on Au /CeO (93%)
1
2
2
obtained at a much higher conversion (51.9%), but it is still
slightly lower than that (98%) obtained at lower conversion
2
1
2
(
15.9%), suggesting that in addition to lattice oxygen, the
-1
strength of aldehyde adsorption may influence the selectivity as
well. This result further confirms the contribution of the weak
adsorption of benzaldehyde on Au /CeO to the high selectivity
1 2
2
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Angewandte Chemie International Edition
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COMMUNICATION
and also suggests that on Au/CeO
directly participates the reaction, thus leading to a much lower
selectivity when the alcohol oxidation involving O
2
-RRCe catalyst gas O
2
oxidation of ethanol to ethanal (steps 7, 8, 9, 10, 11) and the
regeneration of catalyst with the formation of second H
12, 13). Figures S6 and S7 present the energy profile of the first
and second oxidation of ethanol to ethanal on Au /CeO as well
2
O (steps
2
.
1
2
as the optimized structures of corresponding intermediates and
transition states, respectively. On the basis of our calculations,
step 9 in the second oxidation of CH CH OH has the highest
3 2
energy barrier among the elementary steps, which is the rate-
determining step (RDS). The energy barrier of step 9 is 0.79 eV,
indicating the high activity of selective oxidation of alcohol on
Au /CeO . As hinted by the experiments, our calculations indeed
1 2
reveal that lattice oxygen atoms participate in the oxidation
process, giving rise to the ultra-activity. The formation energy of
oxygen vacancy for Au
for a model NP-catalyst Au10/CeO
activity of lattice oxygen in Au /CeO
Further electronic structure analyses indicate that for Au
1
/CeO
2
(0.36 eV) is much less than that
(2.32 eV), indicating higher
than in Au/CeO -NP.
2
/CeO ,
2
Figure
2
The proposed reaction pathways for ethanol oxidation on the
catalyst.
1
2
2
1
Au /CeO
2
1
it is the single-atom Au, rather than the lattice Ce, that acts as
the electron acceptor in the O vacancy formation process,
The presence of metal can effectively activate the lattice
oxygen either through the spillover effect or by weakening M-O
bonding in metal oxide. Therefore, singly dispersed SACs can
maximize the interfacial sites and activation of lattice oxygen.
More lattice oxygen is capable to participate in the oxidation
leading to the rather low O vacancy formation energy. Similar
[
11]
[12]
conclusion can be found in Pt
1
/CeO
2
and Pd
1
/CeO
2
.
In
addition, the desorption energies for CH
3
CHO in the two
oxidation processes are 0.70 eV and 0.50 eV, respectively,
which are low enough to be overcome easily under reaction
temperature and suggest the high selectivity of ethanal on
1 2 2
process on Au /CeO than that on Au/CeO -RRCe catalyst. An
isotope experiment was performed to verify this point, where
1
8
ethanol (CH
to operate in a fix-bed model and the catalytic performance of
Au /CeO for linear aliphatic alcohol oxidation was good as well
Table S2, entry 4 and 5). The results showed clearly that on
Au /CeO sample much more lattice O participated in the
reaction as much more H
RRCe, Figure S4a. In addition, the higher concentration of H
obtained on Au/CeO -RRCe at the initial state may suggest that
directly participated in the reaction, which is in line
with the above experiment result. After a long time reaction (e.g.,
3
CH
2
OH) oxidation by
O
2
was adopted as it is easy
Au
1
2
/CeO , which is in line with the experiment.
80
120
100
1
2
100
80
conversion
selectivity
70
a
(
b
60
80
1
2
50
restarted after
a 12-hour
break
1
2
6
60
4
0
60
conversion
selectivity
2
O was obtained than on Au/CeO -
18
30
4
2
0
0
2
O
40
20
0
2
10
0
20
1
8
0
100
200
300
400
500
some
O
2
1
2
3
4
5
Reaction time (h)
Cycle number
1
8
Figure 3. Conversion and selectivity of benzyl alcohol oxidation over
Au /CeO catalyst in (a) recycle and (b) long-term test. Reaction conditions:
2
of H
500 s and 5500 s), the production of H
2
O had exceeded that
1
6
1
2
2
O on the two samples, respectively, suggesting the active
lattice O were mostly consumed and replenished by
1
6
18
(a) 150 mg catalyst, 5 mL benzyl alcohol, PO
2
= 0.5 MPa, 150 °C, 24 h; (b)
2
O . We
o
-1
2
50 C, 1.1 g catalyst, liquid benzyl alcohol flow rate 0.005 mL min , air flow
then purged the reaction system with helium at the reaction
temperature for 30 min and introduced the mixture of ethanol +
-
rate 20 mL min
1
6
normal
Au /CeO
while on Au/CeO
2
O to the reactor. As shown in Figure S4b, on
1
6
18
Given the recent findings that SACs can be structurally
1
2
both products of H
-RRCe sample much more H
O, further confirming that on Au/CeO
participates in the reaction directly.
2 2
O and H O are now similar
[8a, 8b]
1
6
stable and highly durable during reactions,
we further tested
2
2
O is produced
1
8
16
the reusability of the Au /CeO sample during recycle reaction.
1
2
than H
2
2
-RRCe gas
O
2
The results showed that within 5 cycles both conversion and
selectivity did not change detectably (Figure 3a). The HAADF-
STEM images (Figures 1b and S1B) revealed that no
aggregation of Au atoms to Au clusters or NPs occurred. As in
To understand the reaction mechanism and the
remarkable role of lattice oxygen atoms in selective oxidation of
alcohol on single-atom Au /CeO catalyst, periodic density
1 2
[
4]
general gas-phase oxidation is easy to handle, the stability in a
fix-bed was further tested. In an in-total 500-hour test, both high
conversion (controlling to below 100% to avoid activity
saturation) and high selectivity remained unchanged after an
initial fluctuation (Figure 3b). The reaction was artificially
stopped after 260-hour test for 12 hours and restarted. It
followed that the catalytic performance was not influenced at all,
indicating an excellent operability in practice.
functional theory (DFT) calculations were carried out. With
ethanol adopted to simplify the calculation, the optimized
structures are presented in Figure S5 and the reaction
1 2
mechanism of selective oxidation of alcohol on Au /CeO SAC is
summarized in Figure 2. The calculated energies of
intermediates and activation barriers of each proposed
elementary step are listed in Table S3. Our calculations show
that the whole reaction involves five steps: the first oxidation of
ethanol to ethanal with two H species adsorbing on surface
The aforementioned results demonstrate clearly that CeO
2
supported Au SAC exhibits much higher activity and selectivity
than Au NP catalysts for solvent-free selective oxidation of
(
steps 1, 2, 3), the formation of H
2
O and oxygen vacancy (steps
4
, 5), the adsorption of O at vacancy site (step 6), the second
2
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Angewandte Chemie International Edition
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COMMUNICATION
alcohols, primarily due to the maximized interfacial sites and
activated lattice oxygen, respectively. The maximized
participation of lattice oxygen in catalysis by using single atoms
Keywords: single-atom • interfacial • alcohol oxidation • lattice
oxygen • solvent-free
[
13]
has been reported recently on supported Pt catalyst.
therefore further studied a Pt /CeO and a Pt/CeO -NP catalyst
with NP in 6.5 nm (Figure S8). The results show that Pt /CeO
exhibits a much higher activity than Pt/CeO -NP sample while
maintains a similar selectivity (Table 1 entry 7 and 8). In addition,
the variance in TOFs between Pt /CeO and Pt/CeO -NP is
different from that between Au /CeO and Au/CeO -NP. The
activity is normalized with interfacial sites of Pt/CeO -NP (TOF of
3110 h , see SI for the calculation details) and is close to that
of the Pt /CeO sample. This result is consistent with the recent
We
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12a, 14]
in CO oxidation with M
1
/CeO
2
(M = Au, Pt).
The normalized
-RRCe is,
7
-1
TOF (~8230 h ) with interfacial sites of Au/CeO
however, much higher than that of Au /CeO (~3000 h ), further
1 2
2
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sites.
The calcluated slope based on this model is -0.9 for
Au catalysts and -1.5 for Pt catalysts, which is similar or close to
that for surface (including interfacial) active sites (-0.9 ± 0.1) and
[
5b]
for interficial sites (-1.9 ± 0.2), respectively, further confirming
above conclusion.
In summary, we have demonstrated that SACs can be
highly active and selective for solvent-free oxidation of alcohol
by molecular oxygen. The maximized interfacial sites in SACs
make them much more active than the NP catalyst. The lattice
oxygen activated at the interfacial sites is more selective than
the molecule oxygen activated on the metal surface due to
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high selectivity. This work verifies experimentally that SACs can
maximize the interfacial sites, thus significantly improving the
activity and selectivity. This finding provides a new avenue to
considerably advance the catalytic performance by fabricating
suitable SACs for alcohol oxidation and other reactions occurred
at metal-support interface.
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Acknowledgements
This work was supported by National Key R&D Program of
China (2016YFA0202804), the National Natural Science
Foundation of China (21776270, 21590792, 91645203, 2152109,
21606227), the Strategic Priority Research Program of Chinese
Academy of Science (XDB17000000). The calculations were
performed at Tsinghua National Laboratory for Information
Science and the Computational Chemistry Laboratory (Xuetang)
of the Department of Chemistry, Tsinghua University.
This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.201803272
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Oxide supported metal single-atom
catalysts can maximize the metal-
Tianbo Li, Fei Liu, Yan Tang, Lin Li,
Shu Miao, Yang Su, Junying Zhang,
Jiahui Huang, Hui Sun, Masatake
Haruta, Aiqin Wang, Botao Qiao,* Jun
Li,* Tao Zhang
support
interfacial
sites
thus
significantly improving both the
activity and selectivity in solvent-free
selective oxidation of primary alcohol
as these sites are recently found to
be the active sites for this reaction. It
Page No. – Page No.
Maximizing interfacial sites by
single-atom catalyst for solvent-free
selective oxidation of primary
alcohol
provides
a
new
avenue
to
considerably advance the catalytic
performance by fabricating suitable
SACs for reactions occurred at
metal-support interface.
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Title
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