High-Density and Thermally Stable Palladium Single-Atom Catalysts for Chemoselective Hydrogenations

Article abstract of DOI:10.1002/anie.202007707

Single-atom catalysts (SACs) have shown superior activity and/or selectivity for many energy- and environment-related reactions, but their stability at high site density and under reducing atmosphere remains unresolved. Herein, we elucidate the intrinsic driving force of a Pd single atom with high site density (up to 5 wt %) under reducing atmosphere, and its unique catalytic performance for hydrogenation reactions. In situ experiments and calculations reveal that Pd atoms tend to migrate into the surface vacancy-enriched MoC surface during the carburization process by transferring oxide crystals to carbide crystals, leading to the surface enrichment of atomic Pd instead of formation of particles. The Pd₁/α-MoC catalyst exhibits high activity and excellent selectivity for liquid-phase hydrogenation of substituted nitroaromatics (>99 %) and gas-phase hydrogenation of CO₂ to CO (>98 %). The Pd₁/α-MoC catalyst could endure up to 400 °C without any observable aggregation of single atoms.

Full text of DOI:10.1002/anie.202007707

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
International Edition Chemie
www.angewandte.org
Accepted Article
Title: High-Density and Thermally Stable Palladium Single-atom
Catalysts for Chemoselective Hydrogenations
Authors: Yuefeng Liu, Ying Ma, Yujing Ren, Yanan Zhou, Wei Liu,
Walid Baaziz, Ovidiu Ersen, Cuong Pham-Huu, Mark Greiner,
Wei Chu, Aiqin Wang, and Tao Zhang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202007707
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
High-Density and Thermally Stable Palladium Single-atom
Catalysts for Chemoselective Hydrogenations
Ying Ma,[a],[b],+ Yujing Ren,[a],+ Yanan Zhou,[c],[d] Wei Liu, Walid Baaziz, Ovidiu Ersen, Cuong Pham-
[a]
[e]
[e]
Huu, Mark Greiner, Wei Chu, Aiqin Wang, * Tao Zhang, Yuefeng Liu [a],*
[
f]
[g]
[d]
[a],
[a]
Dedication to the late of Prof. Dang Sheng Su.
[
a]
Y. Ma,+ Dr. R. Ren,+ Dr. W. Liu, Prof. Dr. A. Wang,* Prof. Dr. T. Zhang, Prof. Dr. Y. Liu,*
Dalian National Laboratory for Clean Energy
Dalian Institute of Chemical Physics, Chinese Academy of Science
457 Zhongshan Road, Dalian 116023 (China)
E-mail: yuefeng.liu@dicp.ac.cn; aqwang@dicp.ac.cn
+
[b]
[c]
[d]
Y. Ma
University of Chinese Academy of Sciences
Beijing 100049 (China)
Dr. Y. Zhou
Materials Sciences Division, Lawrence Berkeley National Laboratory
Berkeley, CA 94720 (USA)
Dr. Y. Zhou, Prof. Dr. W. Chu
School of Chemical Engineering,
Sichuan University
Chengdu 610065 (China)
[
[
[
e]
Dr. W. Baaziz, Prof. Dr. O. Ersen
Institute de Physique et Chimie des Matériaux de Strasbourg (IPCMS)
UMR 7504 CNRS-University of Strasbourg
23, rue du Loess, 67037 Strasbourg Cedex 08 (France)
f]
Dr. C. Pham-Huu
Institute of Chemistry and Processes for Energy, Environment and Health (ICPEES)
UMR 7515 CNRS-University of Strasbourg
25 rue Becquerel, 67087 Strasbourg Cedex 02 (France)
g]
Dr. M. Greiner
Max Planck Institute for Chemical Energy Conversion
Stiftstrasse 34-36, Mülheim an der Ruhr, D-45470 (Germany)
Supporting information for this article is given via a link at the end of the document. + These authors contributed equally in this work.
Abstract: Single-atom catalysts (SACs) have shown superior activity
and/or selectivity for many energy- and environment-related reactions,
but their stability at high site density and under reducing atmosphere
remains unresolved. Herein, we elucidate the intrinsic driving force of
palladium single atom with high site density (up to 5 wt%) under
reducing atmosphere, and its unique catalytic performance for
hydrogenation reactions. In-situ experiments (including scanning
transmission electron microscopy, near-ambient pressure X-ray
photoelectron spectroscopy, X-ray diffraction and quasi in-situ
synchrotron X-ray adsorption) and theoretical calculations reveal that
Pd atoms tend to migrate into the surface vacancy-enriched
molybdenum carbide surface during the carburization process by
transferring oxide crystals to carbide crystals, leading to the surface
Introduction
Single-atom catalysts (SACs), with unique geometric and
electronic structures, have shown unrivalled selectivity, superior
activity, and maximized metal atom utilization in a wide sort of
chemical transformations and become a superstar in the past few
years.[1] Owing to the direct chemical bonding of single metal
atoms with the support, the performance of SACs is dependent,
to a great extent, on the nature of the support employed. For
example, when metal single atoms are supported on oxides, the
metal atom coordinates with the oxygen atoms from the support,
pair constitutes the active site.[ Nevertheless,
2]
where the M
1
-O
x
such metal-oxo complex would destruct once the catalyst is
exposed to reducing atmosphere at relatively high temperatures,
which results in aggregation/sintering of single atoms and
consequently loss of their chemoselectivity.[ This scenario is
what the real catalysis is facing in many catalytic processes, such
as selective hydrogenation and other hydro-processing reactions.
In this context, the stability of SACs under reducing atmosphere
has become a bottleneck limiting their practical applications.
Therefore, it is keenly desired to develop a highly stable SAC that
can survive under reducing atmosphere.
1
enrichment of atomic Pd instead of formation of particles. The Pd /α-
MoC catalyst exhibits high activity and excellent selectivity for both
liquid-phase hydrogenation of substituted nitroaromatics (> 99%) and
3]
gas-phase hydrogenation of CO
2
to CO (> 98%), which is far superior
/α-MoC
to their nanoparticles analogues. More importantly, the Pd
1
catalyst could endure harsh reducing/reacting atmosphere up to 400
o
C without any observable aggregation of single atoms. This work
provides an alternative pathway to pursue single-atom catalysts with
preferable performance and stability at high level of site density even
under harsh reducing conditions.
In fact, some strategies have been proposed for the
preparation of thermally stable SACs. For example, by using ceria
1
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
or iron oxide as the trapping center, thermally stable Pt SACs
In order to understand the formation mechanism of Pd
single atoms on the α-MoC surface, we employed in-situ STEM
technique to visualize the transformation process of Pd NPs into
single atoms under carbonizing conditions.[9] The well-defined Pd
nanoparticles of ~ 2.3 nm by colloid method (Fig. S1) were
[
4]
have been achieved . Unfortunately, these SACs cannot survive
under reducing atmosphere; instead, they are only stable under
oxidizing atmosphere due to saturated coordination by the
neighboring oxygen ligands from the support, which in turn render
them catalytically inactive for most reactions.[2a, 3b, 5]
Alternative to oxides, carbides of early transition metals (group
IV-VI) are able to interact strongly with noble metals and thus
provide new opportunity for synthesis of stable SACs.[6] For
example, Pt could be atomically dispersed on α-MoC and the
synthesized and then deposited on the laboratory-made MoO
(named as Pd NPs/MoO hereafter) for in-situ TEM experiments.
3
3
In order to simulate the same carbonization process in the fixed-
bed reactor under atmospheric pressure (1 atm), an
TM
Atmosphere gas holder was used for the in-situ analysis with
resulting Pt
1
/α-MoC SAC was highly active and selective for
the aberration-corrected high-angle annular dark-field STEM
(HAADF-STEM).[10] An area, with both aggregated NPs (~ 9 nm)
and small NPs (~ 3 nm), was selected for in-situ STEM
[
6a]
aqueous phase reforming of methanol
and selective
hydrogenation of substituted nitroarenes. Although the earlier
reported Pt /MoC system with relative low percentage of Pt (< 0.5
[
6c]
1
observation (Figs. 1a, S2 and S3). The MoO
smooth surface at the reaction temperature lower than 300 o
under a mixture of 20% CH in H , maintaining the initial
morphology of Pd NPs/MoO . When the reaction temperature
3
support displays
wt%) could be dispersed as single atoms,[6a] the crucial issue of
nanoparticles disintegrate into single atoms is still unsolved due
to the limitations by scientific instruments. Induced by the
analogous strong metal support interaction (SMSI), Au was found
to disperse as atomic-layered clusters on α-MoC surface and the
strong charge transfer effect empowered the catalyst with high
activity and promising stability for low-temperature water gas shift
reaction.[6d] The re-dispersion of Au particles into the overlayer on
C
4
2
3
increased to 400 oC and 500 oC under the gas mixture, the
support began collapsing and an open-framework structure was
generated. The structure change observed by in-situ STEM at 400
~ 500 oC should be due to the formation of O-rich MoO
composites (Intermediate I in Fig. 1c). Based on in-situ XRD (Figs.
S4 and S5), the MoO was gradually reduced and carbonized
C
y
x
MoC
x
was further revealed during the carburization process by
3
stronger metal-support interaction between Au and carbide
during this process to yield the following structures: MoO
x
(200
materials.[6b] In addition to α-MoC, other transition metal carbides
oC), MoO
C
(350 C) and finally to α-MoC (higher than 650 C).
o
o
x
y
o
(
TMCs) such as WC, NbC and TiC also showed strong interaction
Meanwhile, the Pd NPs began to aggregate at 300 C (Fig. 1b),
and continued increasing the size in horizontal direction to form
the larger particles (marked as 1 in Fig. 1a) with further rise of the
with noble metals and exhibited synergy in various catalytic
transformations although atomic dispersion of noble metals was
not always accomplished.[7] Meanwhile, the TMC-supported
SACs are characterized with dual active sites, which constitute a
o
temperature to 500 C. Breaking change occurred between 600
o
o
C and 700 C, signified by the gradual disappearance of Pd NPs
[
6d,
bi-functional catalyst having a wide spectrum of applications.
x y x y
and the transformation of O-rich MoO C to C-rich MoO C phase
(Intermediates II in Fig. 1c).[11] Finally, all the Pd species were
7
e] [6d, 7e]
However, these features are less explored due to lack of
fundamental understanding of the formation mechanism of SACs
on TMCs.
atomically dispersed together with the formation of α-MoC phase.
The Pd cluster is selected as an example to identify theoretically
5
Herein, we elucidate the formation mechanism of Pd single
atoms on the α-MoC substrate by the state-of-the-art in-situ
techniques, including scanning transmission electron microscopy
the intrinsic driving force for Pd cluster atomization on the Mo-
vacancy enriched α-MoC (200) surface (Fig. 1d). It is found that
an energy barrier of only 0.66 eV is required to overcome the
(
STEM),
near-ambient
pressure
X-ray
photoelectron
5
formation of a Pd single atom from the Pd cluster. The free
spectroscopy (NAP- XPS), X-ray diffraction (XRD) and quasi in-
situ synchrotron X-ray adsorption in conjunction with density
functional theory (DFT) calculations. The Pd nanoparticles (NPs)
are atomically dispersed on the surface vacancy sites on the
energy diagrams (ΔG) clearly show that the dissociative
deposition of Pd atom on α-MoC (200) surface is
thermodynamically favorable, where the ΔG is as high as -2.62
eV. Evidently, the atomic dispersion of Pd on α-MoC is both
thermodynamically and kinetically favorable. The sample after
o
substrate surface at high carbonization temperature (700 C) in
o
CH
4
and H
2
mixture gaseous atmosphere. Our Pd
1
/α-MoC is
carbonization at 700 C was further analyzed by atomic resolution
featured with a high loading of Pd single atoms (up to 5 wt%),
which is mainly attributed to the similar electronic and structural
properties between face centered cubic crystalline α-MoC and Pd
NPs,[7a, 8] as well as ultra-strong interfacial Pd-MoC interaction
energy. One distinct feature of TMC-supported SACs is that they
can endure high-temperature treatment in reducing atmosphere,
which may open up a new avenue to the highly selective and
stable hydrogenation catalysts. In this way, the high selectivity
STEM and corresponding energy dispersive X-ray spectroscopy
(EDS) elemental mapping (Fig. 1e and S6). It is remarkable to
note that not only the Pd element atomically dispersed on the α-
MoC support but also the fcc α-MoC surface crystal shrinks
accordingly, inferring that the Pd atoms are embedded into the
matrix of α-MoC surface (vide infra). For better comparison, we
directly deposited Pd NPs on the α-MoC, the EDS elemental
mapping of as-synthesized Pd NPs/α-MoC sample shows
obvious aggregated Pd X-ray signal based on atomic resolution
STEM images (Fig. S7).
1
and stability of Pd /α-MoC under reducing reaction atmosphere
has been for the first time demonstrated in both liquid-phase
hydrogenation of substituted nitroarenes and gas-phase
2
hydrogenation of CO under relatively harsh reaction conditions.
Results and Discussion
Disintegration of Pd NPs into single atoms
2
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RESEARCH ARTICLE
Figure 1. Structure evolution the dispersion of Pd single atoms under reducing atmosphere. (a) In-situ STEM images recorded the transformation process of
Pd from nanoparticles to atomically dispersed state. (b) The vertical and horizontal size of particle (1) in (a) as a function of carburization temperature. (c)
Scheme illustration showing the size evolution of Pd NPs under different chemical states of Mo-containing compounds. Intermediates I represent the Pd
aggregates on the O-rich MoO
energies along the dissociative pathway of the Pd atom from the Pd
CINEB) method, and the corresponding initial, optimized intermediate and final geometries. (e) STEM image of as-prepared Pd
x
C
y
composites, Intermediates II represents the Pd clusters and atoms on the C-rich MoO
cluster to Pd embedded into defected α-MoC by the climbing image nudged elastic band
/α-MoC sample after
x y
C composites. (d) Calculated free
5
1
(
1
carburization at 700 oC. The measured lattices spacing is 0.21 nm, which is assigned to the α-MoC (200) crystal face. Pd, green; Mo, pink; C, brown. atm:
atmospheric pressure, RT: room temperature.
Table 1. Quasi in-situ EXAFS data fitting results of series of Pd NPs/MoO
3
-T catalyst.a
Samples
Shell
N
R (Å)
r-factor (%)
σ2 (10-2 Å2)
0
ΔE (eV)
Pd Foil
Pd-Pd
Pd-Cl
Pd-Pd
12.0
2.4
3.8
2.74
2.33
2.82
0.4
0.3
0.7
5.3
5.5
5.5
0.02
0.3
-
5% Pd NPs/MoO
3
5
5
5
% Pd NPs/MoO
% Pd NPs/MoO
% Pd NPs/MoO
3
3
3
-300
-500
-700
Pd-Pd
10.1
10.0
1.7
2.73
2.73
2.13
2.78
0.7
0.7
1.1
1.1
-1.3
-1.1
3.3
3.3
2
0.02
0.2
0.5
-
Pd-Pd
Pd-C
Pd-Mo/Pd
8.0
[
a] N, the coordination number for the absorber-backscatterer pair. R, the average absorber-backscatterer distance. σ , the Debye-Waller factor. ΔE
0
, the inner
2
potential correction. The accuracies of the above parameters are estimated as N, ± 20%; R, ± 1 %; σ , ±20%; ΔE
k-space (Δk) and R-space (ΔR) are 3.0–11.0 Å-1 and 1.2–3.1 Å, respectively.
0
, ±20%. The data ranges used for data fitting in
Consistent with the in-situ STEM result, XAS spectra at Pd
K-edge provides complementary evidence for the structural
of Pd NPs observed in the STEM analysis. Remarkable change
occurred with the 700 C -treated sample, with the emergence of
o
change from Pd NPs/MoO
% Pd NPs/MoO , both Pd-Cl and Pd-Pd coordination were
present (Table 1 and Fig. S8a). When the sample was treated in
3
to Pd
1
/α-MoC. For the as-synthesized
Pd-C and Pd-Mo shells. The Pd-C shell has a coordination
number (CN) of 1.7 at distance of 2.13 Å. The Pd-Mo shell has a
coordination number of 8.0 at 2.78 Å, which is shorter than the
Mo-Mo distance (3.00 Å) in the α-MoC, demonstrating the
structure shrinkage upon incorporation of Pd atoms. In addition,
although Pd and Mo signals are indistinguishable in EXAFS due
to their close atomic numbers, the different observations, i.e.
5
3
o
o
4 2
20% CH /H at 300 C and 500 C, the Pd-Cl disappeared and
only Pd-Pd coordination was found. Moreover, the coordination
number increased from 3.8, in the as-synthesized sample, to 10.0
o
in the 500 C -treated one, which is well consistent with the growth
3
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RESEARCH ARTICLE
concomitant appearance of Pd-C shell with the Pd-Mo shell, the
relatively high coordination number (if Pd disperses as single
layer, the CN would be much lower), as well as the disappearance
of Pd NPs in STEM observations, allow us to unambiguously
assign this shell to the formation of Pd-Mo instead of Pd-Pd. On
the other hand, the XANES spectra reflect the change of oxidation
state of Pd during the carburization process. Compared with Pd
3
foil, the absorption edge of fresh Pd NPs/MoO catalyst shifted
towards higher photon energy (Fig. S8b), consistent with the
presence of Pd-Cl coordination in the EXAFS. When the
o
carburization temperature increases to 300 and 500 C, the
absorption edge decreases and becomes rather close to Pd foil,
reflecting the reduction of Pd-Cl complex to zero Pd. However,
when the carburization temperature reaches 700 oC, the
absorption edge is slightly shifting to higher photo energy,
suggesting the electron transfer from Pd to the neighboring C
atoms. To corroborate such electronic interaction between Pd and
α-MoC, we performed NAP-XPS for 5% Pd/MoC sample in 2.0
Figure 2. Structure characterization of Pd SACs at high site density. (a) XRD
o
mbar of 20% CH
passivated sample contains both MoO
and MoC (228.2 eV),[13] the heating treatment at 590 C with 20%
4
/H
2
at 590 C (Figs. S9-S11). While the
patterns of α-MoC and Pd
crystal α-MoC and metallic fcc Pd are plotted as references. (b) Atomic
resolution HAADF-STEM image of 1% Pd /α-MoC. The corresponding fast
1
/α-MoC with different Pd mass loadings. Patterns of
(Mo 3d BE 232.0 eV)[
12]
x
o
1
4 2
CH /H results in the exclusive presence of MoC. At the same
Fourier transform (FFT) pattern and phase contrast profile are also provided. (c)
time, the slightly oxidized Pd species are reduced. However, the
Contour plot of the lattice distances of α-MoC (111) derived from HAADF-STEM
0
Pd 3d peak shifts to higher BE than the typical Pd (335.8 eV vs.
images and the different Pd
Pd /α-MoC with atomic resolution carried out on the EDS with dual-detectors
100 mm energy collection area*2, Hitachi HF5000 microscope), suggesting Pd
1
/α-MoC catalysts. (d) Elemental mapping of 1%
3
35.1 eV), indicating Pd is positively charged.[14] Consistently, the
1
lower binding energy of C 1s (283.2 eV) indicates the electron
transfer from Pd to C instead of Mo, which is in good agreement
with our DFT calculation result (vide infra).
2
(
is homogeneously dispersed on the α-MoC matrix.
Chemical structure of Pd
1
/α-MoC at high site density
The superior capability of α-MoC host matrix for stabilizing
Pd single atoms is further investigated by DFT calculations (Fig.
a). The optimized structure of Mo-vacancy of α-MoC (200)
surface is shown in Figs. S19a and S19b, where the vacancy
interacts with 2 (2C) and 4 (4C) carbons, respectively. It should
be noted that the formation energy (E ) of C-vcacancy on the α-
f
MoC surface is much higher than that of Mo-vcacancy (3.34 eV
vs 1.50 eV, Fig. S20), demonstrating the Mo-vacancy on the α-
The Pd /α-MoC catalysts with Pd loadings ranging from 0.5
1
wt% to 5 wt% were obtained by carbonization of Pd/MoO
3
in 20%
3
o
CH
4
/H
2
mixture at 700 C (for synthesis details see Supporting
Information). The XRD patterns (Fig. 2a) exhibit only pure fcc
crystal phase of α-MoC, there is no any reflections of Pd crystallite
for all of the Pd-containing samples, indicating Pd species is
highly dispersed. Consistently, STEM images did not show any
particles or clusters of Pd except for the α-MoC phase (Fig. 2b,
for additional evidences see Figs. S12-S16), confirming the high
dispersion of Pd species on the α-MoC NPs. Moreover, detailed
MoC surface is thermodynamically favorable. Pd
) are positioned at the above Mo vacancies (Fig. 3b and Figs.
S19c, d), and the binding energy (BE) was calculated. As shown
in Fig. 3a, the relative BE of Pd clusters decorating the Mo-
n
clusters (n = 1-
8
1
investigation of the α-MoC crystal structure in different Pd /α-MoC
n
samples (Fig. 2b, c and Supplementary Figs. S12-S16) reveals
that the lattice distance of the (111) crystal plane of α-MoC
decreases from 0.246 nm to 0.234 nm as the Pd loading
increases from 0 wt% to 5 wt%. Such remarkable lattice shrinkage
is presumably caused by the incorporation of Pd atoms into the
lattice of surface α-MoC. Considering the similar electronic and
structural properties between α-MoC (fcc) and Pd (fcc),[7a, 8] the
incorporated Pd atoms probably occupy the Mo sites, resulting in
the unsaturated coordination of the surface with defects in the as-
prepared catalysts. Similar phenomena have also been
vacancy of α-MoC (200) surface decreases from -5.13 to -3.67 eV
with an increase of n from 1 to 8 for the Pd-4C model and from -
5
.23 to -3.91 eV for the Pd-2C model.[16] In particular, the BE of
Pd
value, which is about 1.0 eV higher than Pd
This result strongly suggests that the Mo-vacancy on the α-MoC
200) surface is able to stabilize the Pd single atoms and the Pd
1
in both Pd-4C and Pd-2C models is maintained at the highest
2
and Pd clusters.
3
(
species prefer being dispersed as single atoms when adsorbed
on the Mo-vacancy enriched α-MoC (200) surface.[7d] The
differences of charge density and the partial density of states
[6d,
previously reported for Au-ZnO and Au-MoC catalytic systems.
(
PDOS) with d orbital of Pd and Mo, together with the p orbital of
1
5]
However, the close atomic number between Pd and Mo does
the neighboring C atoms are also calculated. It shows that the
hybridization between d orbital of Pd and p-orbital of C decreases
with an increase of n (Figs. 3b and S21), suggesting the strongest
orbital interaction when Pd is in the form of single atom. Moreover,
the different charge density (Fig. 3b and S22) shows that the
charge transfer occurs from Pd atom to the α-MoC, and the
localization of the transferred charge is mainly on the neighboring
C atoms. The Bader charge analysis (Table S1) also shows the
not allow the unambiguous identification of Pd single atoms by Z-
contrast STEM imaging (Figs. 1e and 2b). In order to differentiate
Pd and Mo with atomic resolution, we employed EDS with
symmetrically opposed dual-detectors to perform elemental
mapping, which allow us to collect the low X-ray yield specimens
over atomic resolution STEM images. The EDS elemental
mapping of Pd
1
/α-MoC samples clearly display the uniformly
dispersed Pd atoms on α-MoC NPs (Figs. 2d, S17 and S18).
charge transfer from Pd atom to α-MoC, and the Q
M
(charges
transferred from Pd atoms to substrate obtained by Bader charge
4
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
analysis) for Pd
1
/α-MoC is significantly higher than the other
nanoparticles (NPs) supported on α-MoC (Pd NP/α-MoC-EG),
Pd /α-MoC systems. All these theoretical results strongly indicate
n
2 3 2 3
active carbon (1% Pd/C) and γ-Al O (1% Pd/Al O ) (Fig. S28-30),
that Pd single atoms can be stabilized at the Mo vacancies of the
α-MoC surface through orbital interaction and charge transfer
between Pd and the neighboring C atoms. More importantly, such
a SMSI effect can be further explored for synthesis of SACs of
were also tested for comparison. As shown in Fig. 4b, the Lindlar-
type Pd catalyst gave 91% of 4-chloroaniline (4-CA) selectivity
with relatively low 4-NCB conversion (67%), whereas side
reaction occurred over supported Pd NPs resulting in unsatisfied
noble metals other than Pd, such as 1% Ru
MoC, bimetallic 1% Pd -1% Ru /α-MoC (Figs. S23-S27),
demonstrating its generality.
1
/α-MoC, 1% Rh
1
/α-
1
selectivity to 4-CA (20% ~ 56%). By contrast, the Pd /α-MoC
1
1
SACs gave almost 100% selectivity to the target product even at
total conversion of 4-NCB or after extending the reaction time (Fig.
4b and 4c). In addition to 4-NCB, a spectrum of substituted
nitroarenes were all well tolerated, giving > 98.5% selectivity at
100% conversion (Tables S2 and S3). The conversion rate in
terms of per molar of Pd atoms per hour (turnover frequency,
We further calculated the surface density of Pd atoms in the
Pd
1
/α-MoC catalysts with different Pd loadings based on the
specific surface area and exact Pd mass content (Fig. 3c, d and
[
17]
calculation details see Supporting Information).
The surface
/α-MoC is 0.8 atom·nm , which is much lower
than the upper limit corresponding to the single Pd layer on the
-2
density of 2% Pd
1
1
TOF) on different Pd /α-MoC SACs was calculated and it turned
-1
out to be almost the same (~ 2280 h ) irrespective of the Pd
loadings, confirming Pd single atoms function as the active sites
-2
(
111) and (200) crystal plane of α-MoC (4.2 and 2.7 atom·nm for
(
111) and (200) crystal plane, respectively, Fig. 3c). It means that
1
(Table S4). Moreover, the Pd /α-MoC SACs could be used and
the maximum loading of atomic dispersed Pd on α-MoC surface
recycled for at least five times without any loss in activity and
chemoselectivity (Fig. 4d). The HAADF-STEM images of the used
catalyst indicate the absence of any aggregation, demonstrating
its outstanding stability during the reaction (Fig. S31).
-2
is approximately about 5 wt% (2.68 atom·nm ), which agrees well
with our experimental result. XPS analysis shows that the surface
Pd content is 2~3 times higher than the bulk value (Fig. 3d),
indicating that the Pd atoms are predominantly located at the α-
MoC surface, especially at lower metal loadings.
4-chloroaniline
Aniline
a
b
4-nitrochlorobenzene
+H
2
1
00
100
90
6
5
5
4
4
3
3
.0
.5
.0
.5
.0
.5
.0
a
b
4-NCB
Reactant
4-CA
Target product
Pd atom interacted with 2 C atoms (Pd-2C)
Pd atom interacted with 4 C atoms (Pd-4C)
75
50
25
0
H
2
- Cl
H
2
- Cl
80
Pd-2C model
Pd-4C model
+H
2
70
By-product
By-product
60
O
3
d/C Pd/C
2O 3
-EG
C
C
C
/CaC
d/Al
/α-Mo
P
Mo
1
2
3
4
5
6
7
8
C
-
-Mo
b
1%P
P
Mo
d
1
d
1/α
1/α
d-P
%
-
P
d
Number of Pd atoms
1
/α
P
P
P
0.5%
%
1
%
Pd N
1
1
0
c
d
Max Pd Single atoms
[ I ]
c
d
b2
b1
b3
Conversion
Selectivity
4
3
2
1
0
100
80
60
40
20
0
(
111)
100
80
a
8
6
4
2
0
Bulk loading
Surface loading
a
[
I I ]
a
a
a
60
b4
4
2
0
0
0
0
50 100 150 200 250 300
Time (min)
0
.5% 1%
Pd1/a-MoC catalyst
2%
5%
a
a
1
2
3
4
5
Cycles
Figure 3. Simulations of the high density of Pd single atoms on the α-MoC surface.
a) Pd clusters (n = 1~8) decorating Mo-vacancy in α-MoC (200) surface with
Figure 4. Chemoselective hydrogenation of 4-nitrochlorobenzene over
different Pd catalysts.(a) The possible reaction pathways for the
hydrogenation of 4-NCB by using Pd-containing catalysts. (b) Conversion
and product selectivityover various Pd-containing catalysts. (c) Conversion
of 4-NCB and target product selectivity as a function of reaction time over
(
n
absolute values of binding energies. (b) Structure models of Pd atom bonded into
lattice of (200) crystal phase of α-MoC, showing the two possible positions where
the Pd atom is interacted with 2 and 4 carbon atoms, respectively. (c) The unit cell
(c1, c2) and (111), (200) crystal face (c3, c4) of fcc α-MoC structure. The blue
the 1% Pd
1
/α-MoC catalyst; (d) Recycling tests for 4-NCB hydrogenation
triangle and square in (c1, c2) are representative to (111) and (200) crystal faces,
respectively. The grey areas in (c3, c4) are representative to blue sections in (c1,
c2) respectively. (d) Exactly Pd mass loading and the atomic density of Pd on the
over the 1% Pd
1
/α-MoC catalyst.
The superior chemoselectivity of Pd
originates from its preferential adsorption for -NO
1
/α-MoC SAC
over other
2
1
various Pd /α-MoC catalysts. Dash-dot line [I] and [II] represent the maximum Pd
functional groups (for detail see Fig. S32 and Tables S5-S6) due
to the unique feature of isolated Pd sites,[20] as confirmed by
single atoms on the surface of (111) and (200) crystal face of α-MoC, respectively.
attenuated total reflection infrared spectra (ATR-IR), which had
been well discussed in the recent work by Pérez-Ramírez and co-
Chemoselective hydrogenation of Pd SACs
Since Pd
1
/α-MoC SAC is obtained under high temperature
workers.[20d] Meanwhile, the Pd
higher adsorption heat and H
1
/α-MoC catalyst exhibited both
uptake compared to α-MoC support,
reducing atmosphere, it is expected that it can be efficiently used
for selective hydrogenation processes.[18] First, the catalyst was
evaluated for the chemoselective hydrogenation of 4-
nitrochlorobenzene (4-NCB) in liquid phase under milder
2
which results in the satisfying hydrogenation activity (Figure S33).
Stability under harsh reducing conditions
conditions (60 oC, 5 bar H
) (Fig. 4a). Blank test with α-MoC
2
1
To further evaluate the stability of Pd /α-MoC SACS Catal.
showed that the support itself was barely active under such mild
conditions. Typical Pd catalysts, such as commercial Lindlar-type
Pd catalyst (with harmful Pb modified),[19] Pd/C catalyst and Pd
yst, we then tested it for reverse water gas shift reaction (RWGS,
o
CO
2
+ 3H
:H
2
 CO + 2H
2
+ H
2
O) at 400 C in a feed gas consisting
(Fig. 5a).[21] Under
of CO
2
2
= 1:3 to produce syngas from CO
2
5
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Angewandte Chemie International Edition
10.1002/anie.202007707
RESEARCH ARTICLE
such relatively high H
displays a high selectivity towards CO (98.2%, CO/CH
3.7) and rather high stability during 50 h time-on-stream (Fig. 5b,
c). In contrast, the CO selectivity over the bare α-MoC support,
Pd NP/α-MoC-EG and 1% Pd/Al NPs catalyst are much lower
than Pd /α-MoC SACs catalyst. (Fig. 5c). Again, the HAADF-
2
partial pressure conditions, the catalyst
provides a novel route for developing highly stable single-atom
catalysts in harsh reducing conditions.
4
ratio of
5
2
O
3
Experimental Section
1
STEM images and related EDS mapping images of the used
catalyst showed Pd was well dispersed as exclusive single atoms
after the reaction (Figs. 5d, e and S34). Compared with those
oxide-supported SACs that could not endure relatively harsh
Experimental procedures, including catalyst preparation,
characterizations, in-situ experiments (STEM, XRD, NAP-XPS),
DFT calculations and reaction tests are provided in the supporting
information. In brief, the Pd
from 0.5 wt% to 5wt% were synthesized by using the certain
amount of palladium nitrate dehydrate (Pd (NO ) or colloidal Pd
NPs supported on the home-made MoO
at 700 oC in the
presence of 20% CH /H gas mixture. The liquid-phase
1
/α-MoC catalysts with Pd mass range
reducing environment,[22] our Pd
/α-MoC SACs exhibited the
1
highest stability against aggregation so that the high selectivity of
3 2
)
CO could be maintained for long duration at high temperatures.
3
o
Even though the Pd
pure H
1
/α-MoC SAC was treated at 400 C under
4
2
2
atmosphere, the single-atom dispersion could be
hydrogenation of substituted nitroarenes was performed in the
autoclave reactor with pressure control systems under the
maintained well (Figs. 5f and S35), demonstrating the outstanding
stability of such catalyst in harsh reducing atmosphere.
following reaction conditions: 5 bar of H
C, metal/substrate = 0.37 mol%, 0.5 mmol substrate, 5 mL
2
, reaction temperature 60
High T (≥ 400 oC)
c
60
a
CO + 2H2 (syngas, Target)
CH4 (By-product )
53.7
o
CO2 + 3H2
00
5
0
reaction mixture. The gas-phase hydrogenation reaction of CO
2
1
b
40
(reverse water gas shift reaction) was conducted on the fix-bed
quartz tube reactor (d = 8 mm) using 40 mLmin-1 of feed gas
1%Pd1/α-MoC, CO Sel.
80
60
40
20
0
3
2
1
0
0
0
0
1
%Pd/g-Al2O3, CO Sel.
24.8
(CO
2
:H
2
=1/3). Prior to the reaction, the Pd
1
/α-MoC was reduced
at 400 oC in 20% CH
/H gas mixture. DFT calculations were
4
2
1%Pd /α-MoC, CO2 Conv.
1
6
.5
performed using the Vienna ab initio simulation package (VASP)
code. Projector augmented wave (PAW) was used to describe
the interactions of the core electrons between different atoms.
Generalized gradient approximation (GGA) in the form of the
Perdew-Burke-Ernzerhof (PBE) functional was accounted to
describe the exchange-correlation interactions.
2.7
1%Pd/g-Al2O3, CO2 Conv.
C
-M -MoC- d/Al2
o
C
EG O3
o
-M
a
/
a
/a
NP
Pd
0
10
20
30
40
Pd1
1
%P
1
%
Time on stream (h)
e
Pd
d
f
1
nm
Mo
Acknowledgements
1
nm
1 nm
This work was financially supported from the NSFC of China
(
21972140, 91645117 and 21872144), Strategic Priority
Research Program of the Chinese Academy of Sciences
XDB17020100) and the National Key Projects for Fundamental
Figure 5. Stability of Pd SACs under harsh reducing atmosphere. (a) The
possible reaction pathways for reverse water gas shift reaction (RWGS) for
(
the production of synthetic gas (syngas). (b) The stability of the 1% Pd
MoC and 1% Pd/Al
catalysts for RWGS reaction under 400 oC in
CO :H =1:3. (c) The average CO/CH ratio for 1% Pd /α-MoC, initial α-MoC
support, Pd NPs/α-MoC-EG and 1% Pd/Al . STEM-HAADF images and
corresponding EDS elemental mapping of 1% Pd /α-MoC catalyst (d, e) after
RWGS reaction and (f) after 400 oC reduction by H
, in which the Pd single
1
/α-
Research and Development of China (2016YFA0202801),
LiaoNing Revitalization Talents Program (XLYC1907053), CAS
Youth Innovation Promotion Association (2018220) and Talents
Innovation Project of Dalian city (2016RD04 and 2017RQ032).
We also thank the BL14W beamline at the Shanghai Synchrotron
Radiation Facility. Dr. Hiroaki Matsumoto is acknowledged for
performing a part of atomic resolution STEM-HAADF and EDS
mapping analysis at probe corrected Hitachi HF5000 microscope.
The authors also acknowledge Prof. Robert Schlögl (FHI, Berlin)
and Prof. Linwang Wang (LBNL, Berkeley) for fruitful discussion
during the preparation of the manuscript and DFT results.
2 3
O
2
2
4
1
2 3
O
1
2
atoms are still presented in the sample after high temperature
reaction/reduction.
Conclusion
In summary, we report a highly stable and selective single-
Keywords: Single-atom catalysis • palladium • strong metal-
support interaction • chemoselective hydrogenations • thermal
stability
atom catalyst Pd
functionalized nitroarenes at mild conditions and gas-phase
hydrogenation of CO at relatively harsh conditions. The superior
1
/α-MoC for both liquid-phase hydrogenation of
2
stability of Pd single atoms against aggregation under reducing
atmosphere can be ascribed to the strong electronic interaction
with α-MoC through orbital interaction and charge transfer
between Pd and the neighboring C atoms, which favors Pd single
atoms to occupy the Mo vacancy on the α-MoC. The strategy for
stabilizing noble metal single atoms on the α-MoC surface
[
1] a) B. Qiao, A. Wang, X. Yang, L. F. Allard, Z. Jiang, Y. Cui, J. Liu,
J. Li, T. Zhang, Nat. Chem. 2011, 3, 634; b) M. Yang, S. Li, Y.
Wang, J. A. Herron, Y. Xu, L. F. Allard, S. Lee, J. Huang, M.
Mavrikakis, M. Flytzani-Stephanopoulos, Science 2014, 346,
1498-1501; c) P. Liu, Y. Zhao, R. Qin, S. Mo, G. Chen, L. Gu, D.
M. Chevrier, P. Zhang, Q. Guo, D. Zang, Science 2016, 352, 797-
800; d) L. Liu, A. Corma, Chem. Rev. 2018, 118, 4981-5079; e) A.
Wang, J. Li, T. Zhang, Nat. Rev. Chem. 2018, 2, 65; f) X. Cui, W.
6
This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.202007707
RESEARCH ARTICLE
Li, P. Ryabchuk, K. Junge, M. Beller, Nat. Catal. 2018, 1, 385; g)
L. Cao, W. Liu, Q. Luo, R. Yin, B. Wang, J. Weissenrieder, M.
Soldemo, H. Yan, Y. Lin, Z. Sun, Nature 2019, 565, 631; h) L.
Zhang, M. Zhou, A. Wang, T. Zhang, Chem. Rev. 2019; i) Z. Li, S.
Ji, Y. Liu, X. Cao, S. Tian, Y. Chen, Z. Niu, Y. Li, Chem. Rev. 2019,
Almora-Barrios, N. r. Lꢁpez, J. Perez-Ramirez, ACS Catal. 2015,
5, 3767-3778.
[21]Y. Ma, Z. Guo, Q. Jiang, K.-H. Wu, H. Gong, Y. Liu, J. Energy
Chem. 2020, 50, 37-43.
[22]a) J. H. Kwak, L. Kovarik, J. n. Szanyi, ACS Catal. 2013, 3, 2094-
2100; b) J. H. Kwak, L. Kovarik, J. n. Szanyi, ACS Catal. 2013, 3,
2449-2455; c) M.-M. Millet, G. Algara-Siller, S. Wrabetz, A.
Mazheika, F. Girgsdies, D. Teschner, F. Seitz, A. Tarasov, S. V.
Levchenko, R. Schlꢂgl, J. Am. Chem. Soc. 2019, 141, 2451-2461.
120, 623-682.
[
[
2] a) P. Liu, N. Zheng, Natl. Sci. Rev. 2018, 5, 636-638; b) N.
Daelman, M. Capdevila-Cortada, N. López, Nat. Mat 2019, 18,
1215-1221; c) Y. Ren, Y. Tang, L. Zhang, X. Liu, L. Li, S. Miao, D.
S. Su, A. Wang, J. Li, T. Zhang, Nat. Commun. 2019, 10, 1-9.
3] a) M. Moliner, J. E. Gabay, C. E. Kliewer, R. T. Carr, J. Guzman,
G. L. Casty, P. Serna, A. Corma, J. Am. Chem. Soc. 2016, 138,
15743-15750; b) L. DeRita, J. Resasco, S. Dai, A. Boubnov, H. V.
Thang, A. S. Hoffman, I. Ro, G. W. Graham, S. R. Bare, G.
Pacchioni, Nat. Mater. 2019, , 18, 746-751.
[
4] a) J. Jones, H. Xiong, A. T. DeLaRiva, E. J. Peterson, H. Pham,
S. R. Challa, G. Qi, S. Oh, M. H. Wiebenga, X. I. P. Hernández,
Science 2016, 353, 150-154; b) R. Lang, W. Xi, J.-C. Liu, Y.-T.
Cui, T. Li, A. F. Lee, F. Chen, Y. Chen, L. Li, L. Li, Nat. Commun.
2019, 10, 234.
[
[
5] K. Ding, A. Gulec, A. M. Johnson, N. M. Schweitzer, G. D. Stucky,
L. D. Marks, P. C. Stair, Science 2015, 350, 189-192.
6] a) L. Lin, W. Zhou, R. Gao, S. Yao, X. Zhang, W. Xu, S. Zheng, Z.
Jiang, Q. Yu, Y.-W. Li, Nature 2017, 544, 80; b) J. Dong, Q. Fu,
Z. Jiang, B. Mei, X. Bao, J. Am. Chem. Soc. 2018, 140, 13808-
13816; c) L. Lin, S. Yao, R. Gao, X. Liang, Q. Yu, Y. Deng, J. Liu,
M. Peng, Z. Jiang, S. Li, Nat. Nanotechnol. 2019, 14, 354; d) S.
Yao, X. Zhang, W. Zhou, R. Gao, W. Xu, Y. Ye, L. Lin, X. Wen, P.
Liu, B. Chen, Science 2017, 357, 389-393.
[
7] a) S. Oyama, Catal. Today 1992, 15, 179-200; b) A. L. Ivanovskii,
Russ. Chem. Rev. 2009, 78, 303; c) T. G. Kelly, J. G. Chen, Chem.
Soc. Rev. 2012, 41, 8021-8034; d) S. T. Hunt, M. Milina, A. C.
Alba-Rubio, C. H. Hendon, J. A. Dumesic, Y. Román-Leshkov,
Science 2016, 352, 974-978; e) Z. Li, Y. Cui, Z. Wu, C. Milligan,
L. Zhou, G. Mitchell, B. Xu, E. Shi, J. T. Miller, F. H. Ribeiro, Nat.
Catal. 2018, 1, 349; f) J. Zhang, Y. Zhao, X. Guo, C. Chen, C.-L.
Dong, R.-S. Liu, C.-P. Han, Y. Li, Y. Gogotsi, G. Wang, Nat. Catal.
2018, 1, 985; g) Z. Li, L. Yu, C. Milligan, T. Ma, L. Zhou, Y. Cui, Z.
Qi, N. Libretto, B. Xu, J. Luo, Nat. Commun. 2018, 9, 5258.
8] J. G. Chen, Chem. Rev. 1996, 96, 1477-1498.
[
[
[
9] F. Tao, P. A. Crozier, Chem. Rev. 2016, 116, 3487-3539.
10]a) S. Zhang, C. Chen, M. Cargnello, P. Fornasiero, R. J. Gorte, G.
W. Graham, X. Pan, Nat. Commun. 2015, 6, 7778; b) J. C.
Matsubu, S. Zhang, L. DeRita, N. S. Marinkovic, J. G. Chen, G.
W. Graham, X. Pan, P. Christopher, Nat. Chem. 2017, 9, 120.
11]K. T. Jung, W. B. Kim, C. H. Rhee, J. S. Lee, Chem. Mater. 2004,
[
[
[
[
16, 307-314.
12]D. Gazzoli, F. Prinetto, M. Campa, A. Cimino, G. Ghiotti, V.
Indovina, M. Valigi, Surf. Interface Anal. 1994, 22, 398-402.
13]M. D. Porosoff, X. Yang, J. A. Boscoboinik, J. G. Chen, Angew.
Chem. Int. End. 2014, 53, 6705-6709.
14]R. G. Rao, R. Blume, T. W. Hansen, E. Fuentes, K. Dreyer, S.
Moldovan, O. Ersen, D. D. Hibbitts, Y. J. Chabal, R. Schlögl, Nat.
Commun. 2017, 8, 340.
[
[
[
[
[
15]M.-H. Liu, Y.-W. Chen, X. Liu, J.-L. Kuo, M.-W. Chu, C.-Y. Mou,
ACS Catal. 2015, 6, 115-122.
16]A. F. Guillermet, J. Häglund, G. Grimvall, Phys. Rev. B 1992, 45,
11557.
17]D.-S. Kim, J.-H. Yang, S. Balaji, H.-J. Cho, M.-K. Kim, D.-U. Kang,
Y. Djaoued, Y.-U. Kwon, Crystengcomm 2009, 11, 1621-1629.
18]L. Zhang, M. Zhou, A. Wang, T. Zhang, Chem. Rev. 2020, 120,
683-733.
19]a) G. Vilé, N. Almora‐Barrios, S. Mitchell, N. López, J. Pérez‐
Ramírez, Chem. Eur. J. 2014, 20, 5926-5937; b) G. Vilé, D. Albani,
N. Almora ‐ Barrios, N. López, J. Pérez ‐ Ramírez,
ChemCatChem 2016, 8, 21-33; c) G. Vilé, D. Albani, M.
Nachtegaal, Z. Chen, D. Dontsova, M. Antonietti, N. López, J.
Pérez‐Ramírez, Angew. Chem. Int. End. 2015, 54, 11265-
11269.
[
20]a) J. Zhang, L. Wang, Y. Shao, Y. Wang, B. C. Gates, F. S. Xiao,
Angew. Chem. Int. End. 2017, 56, 9747-9751; b) L. Wang, E.
Guan, J. Zhang, J. Yang, Y. Zhu, Y. Han, M. Yang, C. Cen, G. Fu,
B. C. Gates, Nat. Commun. 2018, 9, 1362; c) S. Zhang, C.-R.
Chang, Z.-Q. Huang, J. Li, Z. Wu, Y. Ma, Z. Zhang, Y. Wang, Y.
Qu, J. Am. Chem. Soc. 2016, 138, 2629-2637; d) G. Vilꢀ, N.
7
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RESEARCH ARTICLE
Entry for the Table of Contents
o
Plenty is no plague. The Pd
1
/α-MoC catalyst with high mass loading could endure harsh reducing/reacting atmosphere up to 400 C
without any observable aggregation of single atoms, thus provide a pathway to decouple the hydrogenation activity and stability of
single-atom catalysts.
8
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