Anchoring Positively Charged Pd Single Atoms in Ordered Porous Ceria to Boost Catalytic Activity and Stability in Suzuki Coupling Reactions

Article abstract of DOI:10.1002/smll.202001782

Single-atom (SA) catalysis bridging homogeneous and heterogeneous catalysis offers new opportunities for organic synthesis, but developing SA catalysts with high activity and stability is still a great challenge. Herein, a heterogeneous catalyst of Pd SAs anchored in 3D ordered macroporous ceria (Pd-SAs/3DOM-CeO₂) is developed through a facile template-assisted pyrolysis method. The high specific surface area of 3DOM CeO₂ facilitates the heavily anchoring of Pd SAs, while the introduction of Pd atoms induces the generation of surface oxygen vacancies and prevents the grain growth of CeO₂ support. The Pd-SAs/3DOM-CeO₂ catalyst exhibits excellent activity toward Suzuki coupling reactions for a broad scope of substrates under ambient conditions, and the Pd SAs can be stabilized in CeO₂ in long-term catalytic cycles without leaching or aggregating. Theoretical calculations indicate that the CeO₂ supported Pd SAs can remarkably reduce the energy barriers of both transmetalation and reductive elimination steps for Suzuki coupling reactions. The strong metal-support interaction contributes to modulating the electronic state and maintaining the stability of Pd SA sites. This work demonstrates an effective strategy to design and synthesize stable single-atom catalysts as well as sheds new light on the origin for enhanced catalysis based on the strong metal-support interactions.

Full text of DOI:10.1002/smll.202001782

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Anchoring Positively Charged Pd Single Atoms in Ordered
Porous Ceria to Boost Catalytic Activity and Stability in
Suzuki Coupling Reactions
Xueqin Tao, Ran Long, Di Wu, Yangguang Hu, Ganhua Qiu, Zeming Qi, Benxia Li,*
Ruibin Jiang,* and Yujie Xiong*
1. Introduction
Single-atom (SA) catalysis bridging homogeneous and heterogeneous
catalysis offers new opportunities for organic synthesis, but developing SA
Palladium-catalyzed Suzuki coupling reac-
tions are a class of the most straightfor-
catalysts with high activity and stability is still a great challenge. Herein,
ward and effective pathways for producing
a heterogeneous catalyst of Pd SAs anchored in 3D ordered macroporous
biphenyl compounds which are critical
ceria (Pd-SAs/3DOM-CeO2) is developed through a facile template-assisted
pyrolysis method. The high specific surface area of 3DOM CeO2 facilitates
the heavily anchoring of Pd SAs, while the introduction of Pd atoms induces
the generation of surface oxygen vacancies and prevents the grain growth
of CeO2 support. The Pd-SAs/3DOM-CeO2 catalyst exhibits excellent
activity toward Suzuki coupling reactions for a broad scope of substrates
under ambient conditions, and the Pd SAs can be stabilized in CeO2 in
long-term catalytic cycles without leaching or aggregating. Theoretical
calculations indicate that the CeO2 supported Pd SAs can remarkably
reduce the energy barriers of both transmetalation and reductive
elimination steps for Suzuki coupling reactions. The strong metal-support
interaction contributes to modulating the electronic state and maintaining
the stability of Pd SA sites. This work demonstrates an effective strategy
to design and synthesize stable single-atom catalysts as well as sheds
new light on the origin for enhanced catalysis based on the strong metal-
support interactions.
intermediates for the synthesis of pharma-
ceutical drugs, complex natural products
and functional materials.[1–3] The homo-
geneous catalysts of organometallic Pd-
complexes are traditionally used in these
reactions.[4–6] However, most of the ligands
in the organometallic Pd compounds are
expensive, toxic, or both. Moreover, the
homogeneous catalysts face difficulty in
their recovery and reusability, leading to
the high cost of catalysts and the tedious
procedures in product purification. In
comparison, the heterogeneous catalysts
of Pd clusters or nanoparticles (NPs) on
solid supports may overcome these limi-
tations and thus have been paid much
attention.[7–10] Nonetheless, these hetero-
geneous catalysts usually suffer from less
exposed active sites and limited accessi-
bility for reactants, compared with their
X. Tao, G. Qiu, Prof. B. Li
Department of Chemistry
School of Science
Zhejiang Sci-Tech University
Zhejiang 310018, P. R. China
E-mail: libx@mail.ustc.edu.cn
Prof. R. Long, Dr. D. Wu, Y. Hu, Dr. Z. Qi, Prof. Y. Xiong
Hefei National Laboratory for Physical Sciences at the Microscale
iChEM (Collaborative Innovation Center of Chemistry for Energy
Materials)
Prof. R. Jiang
Key Laboratory of Applied Surface and Colloid Chemistry
National Ministry of Education
Shaanxi Key Laboratory for Advanced Energy Devices
Shaanxi Engineering Lab for Advanced Energy Technology
School of Materials Science and Engineering
Shaanxi Normal University
Xi’an 710119, P. R. China
E-mail: rbjiang@snnu.edu.cn
Prof. Y. Xiong
Institute of Energy
Hefei Comprehensive National Science Center
350 Shushanhu Rd., Hefei, Anhui 230031, P. R. China
School of Chemistry and Materials Science
and National Synchrotron Radiation Laboratory
University of Science and Technology of China
Hefei, Anhui 230026, P. R. China
E-mail: yjxiong@ustc.edu.cn
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.202001782.
DOI: 10.1002/smll.202001782
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homogeneous counterparts. For this reason, exploring the new
catalysts which have both advantages of homogeneous (high
activity) and heterogeneous (high durability and easy separa-
tion) ones is of great significance for Suzuki coupling reactions.
As a new frontier of heterogeneous catalysis, single-atom
(SA) catalysis represents such a new opportunity to link homo-
geneous and heterogeneous catalysis.[11–13] Upon isolating single
metal atoms on the solid supports with high surface area, the SA
catalysts exhibit great advantages in maximizing the metal atom
utilization comparable to that of homogeneous catalysts.[14–16]
Although the TiO2 nanocrystals[17] and exfoliated graphitic carbon
nitride[18] supported Pd SAs have been demonstrated to show
superior activities to homogeneous systems in Sonogashira and
Suzuki C–C coupling reactions under heating conditions and pro-
tection of inert gases, respectively, the great challenges still remain
in elucidating the origin for high catalytic activity and imple-
menting the cross-coupling reactions under ambient conditions.
For the development of heterogeneous SA catalysts, support
material is a non-negligible factor to be considered. The sup-
port not only determines the stability and accessibility of SA
sites but also affects their activity and selectivity due to metal−
support interaction.[19–22] Ideally, ordered porous support with
a high specific surface area can expose abundant active sites
and promote the diffusion of substrate molecules.[23–25] Despite
of a few recent cases for Pd SA catalysts,[26–29] the precise inte-
gration of the support possessing 3D ordered macroporous
(3DOM) structure with the in situ anchored Pd SAs has rarely
been achieved. Ceria (CeO2) is a favorable support material that
can stabilize metal species even at high temperature owing to
the strong metal−support interaction.[30–32] We thus inferred
that such strong metal−support interactions will endow CeO2
with the excellent ability to stabilize Pd SAs through an adap-
tive coordination as well as modify the electronic structures and
chemical properties of Pd atoms, playing key roles in enhancing
overall catalytic performance. Furthermore, creating oxygen
vacancies (OVs) on the surface of CeO2, by taking advantage
of the easy shift between Ce4+ and Ce3+ ions, will offer more
opportunities for reinforcing the stability as well as balancing
the positive charges of Pd SA sites, thus improving the catalytic
activity.[33–35] Taken together, the parameters of oxide supports
including geometric construction, electronic structure, and
defective sites potentially provide additional knobs for tailoring
the overall performance of SA catalysts.
reduce the energy barriers of transmetalation and reductive
elimination steps for the Suzuki reactions. The Pd SAs can be
 
stabilized in 3DOM CeO2 by the strong Pd O Ce bonds for a
long-term reaction, without leaching or aggregating.
2. Results and Discussion
The Pd-SAs/3DOM-CeO2 catalyst is synthesized through a
template-assisted in situ pyrolysis method as illustrated in
Scheme 1. Specifically, polystyrene microspheres are packed
to serve as colloidal crystal templates, which allows the succes-
sive impregnation of Ce and Pd precursors and their further
transformation into CeO2 loaded with Pd SAs around the tem-
plates. Scanning electron microscopy (SEM) and transmis-
sion electron microscopy (TEM) images (Figure 1a,b) reveal
that the Pd-SAs/3DOM-CeO2 catalyst possesses a honeycomb-
like structure composed of well-arranged spherical voids with
diameters of ≈180 nm and honeycomb walls with thickness
of ≈10 nm. The crystal phase in this sample is characterized
as cubic CeO2 (JCPDS No. 81-0792) by X-ray diffraction (XRD)
(Figure S1, Supporting Information), without any diffraction
peak of Pd species. According to the high-resolution TEM
(HRTEM) images (inset of Figure 1b and Figure S2, Supporting
Information), the macropore walls are constructed by CeO2
nanocrystals with average grain size about 4.56 nm. The lattice
fringes with spacing of 0.318 nm belong to the (111) plane of
cubic CeO2 nanocrystals. The granular stacking forms the ran-
domly distributed mesopores in macropore walls, which has
been further confirmed by N2 adsorption-desorption measure-
ment indicating an average pore size of 9.59 nm (Table S1 and
Figure S3, Supporting Information). The specific surface area
and the cumulative pore volume of Pd-SAs/3DOM-CeO2 are
determined to be 150.77 m2 g−1 and 0.29 cm3 g−1, respectively,
which are larger than those (135.15 m2 g−1 and 0.26 cm3 g−1) of
pure 3DOM-CeO2 support with similar honeycomb-like struc-
tures (Figure S4a,b, Supporting Information). HRTEM image
(Figure S4c, Supporting Information) indicates that the CeO2
nanocrystals in pure 3DOM-CeO2 have a larger average grain
size of 7.69 nm. The high-temperature treatment reduces a
little the surface area of pure CeO2 due to the growth of CeO2
grains.[36] In contrast, the introduction of Pd into CeO2 con-
verts the surface O−Ce species to Pd−O−Ce species, which
prevents the grain growth of CeO2 under high temperature,
thus resulting in the larger surface area and pore volume of Pd-
SAs/3DOM-CeO2.[37] The large surface area and high porosity
of Pd-SAs/3DOM-CeO2 are beneficial to the sufficient expo-
sure of Pd-SA active sites and the easy diffusion of substrate
molecules during catalytic reactions. Aberration-corrected
high-angle annular dark-field scanning transmission electron
Herein, the heterogeneous catalyst of Pd SAs anchored in
3DOM structural ceria (Pd-SAs/3DOM-CeO2) has been syn-
thesized through a facile template-assisted in situ pyrolysis
method. This catalyst presents a desirable integration of atomi-
cally dispersed Pd active sites with 3DOM-structural CeO2 sup-
port through two-way interactions. Specifically, the high specific
surface area of 3DOM CeO2 facilitates the anchoring of Pd SAs
with a high loading content of 1.76 wt%, while the introduction
of Pd atoms induces the generation of abundant OVs and pro-
motes the mesoporosity of CeO2 support. The Pd-SAs/3DOM-
CeO2 catalyst exhibits excellent activity and selectivity toward
Suzuki coupling reactions under ambient conditions (298 K
and air atmosphere), remarkably surpassing the homogeneous
Pd(PPh3)4 and the supported Pd clusters or Pd NPs. The supe-
rior performance of Pd-SAs/3DOM-CeO2 is derived from
the positively charged Pd SA active sites which remarkably
Scheme 1. Schematic illustration for the synthesis of Pd-SAs/3DOM-
CeO2 catalyst.
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Figure 1. Electron microscopy characterizations of Pd-SAs/3DOM-CeO2 catalyst. a) SEM, b) TEM and HRTEM (inset), and c) HAADF-STEM images,
d,e) the intensity profiles taken along line 1 and line 2 in (c), f) HAADF-STEM image and EDS elemental mappings.
microscopy (AC-HAADF-STEM) provides the atomic resolution
image (Figure 1c) of Pd-SAs/3DOM-CeO2 sample, in which
some dark dots related to the lower intensity of HAADF-STEM
signal (Figure 1d) are most likely the isolated Pd atoms which
have the smaller atomic number than that of Ce atoms.[29,38]
The elemental mappings (Figure 1f) from energy dispersive
spectroscopy (EDS) confirm the uniform distribution of Ce
and Pd, providing further evidence for the existence of indi-
vidually dispersed Pd atoms in the material. The Pd content
in Pd-SAs/3DOM-CeO2 is determined by inductively coupled
plasma mass spectrometry (ICP-MS) to be 1.76 wt%. Such a
high loading of Pd SAs stabilized by oxide supports has rarely
been obtained (Table S2, Supporting Information), demon-
strating the advantage of 3DOM CeO2 support.
Further insight into the local coordination structure of
Pd atoms in Pd-SAs/3DOM-CeO2 is gained by the element-
selective X-ray absorption fine structure spectroscopy (XAFS).
As shown in Figure 2a, the Fourier-transformed extended XAFS
(FT-EXAFS) of Pd-SAs/3DOM-CeO2 exhibits only one peak at
1.5 Å, corresponding to Pd−O shell in PdO reference. No Pd-Pd
coordination is observed in Pd-SAs/3DOM-CeO2, affirming
that the isolated Pd atoms are anchored in 3DOM CeO2 with
2.00 Å. To look into the local environment of Pd species, we
employ CO-probing diffused reflection infrared Fourier trans-
form spectroscopy (DRIFTS) to characterize Pd-SAs/3DOM-
CeO2, by comparison with two reference samples of H2-treated
Pd-SAs/3DOM-CeO2 (Pd-SAs/3DOM-CeO2-H2) and 3DOM-
CeO2 supported Pd NPs (Pd-NPs/3DOM-CeO2, Figure S5,
Supporting Information), as shown in Figure 2d. Only one
sharp peak at 2153 cm−1 is observed in the DRIFT spectrum
of Pd-SAs/3DOM-CeO2, which belongs to the stretching fre-
quency of linear-adsorbed CO on Pd SAs, confirming the
uniform dispersion of isolated Pd single atoms. In contrast,
Pd-SAs/3DOM-CeO2-H2 displays another broad peak around
2098 cm−1 related to the linear-adsorbed CO on Pd clusters.[29]
The multifarious adsorption sties on Pd clusters result in var-

ious vibrational frequencies of C O bond and thus a broad
peak. This suggests that the H2 treatment of Pd-SAs/3DOM-

CeO2 at 350 °C for 1 h breaks some of Pd O bonds and leads
to the aggregation of Pd atoms into Pd clusters. Moreover, the
stretching frequency peak of the linear-adsorbed CO on Pd SAs
has an obvious blue-shift from that on Pd clusters, indicating
that the isolated Pd SAs are more positively charged than the
Pd clusters.[39] For Pd-NPs/3DOM-CeO2 sample, we observe the
two broad bands centered at 1890 and 2090 cm−1, corresponding
to the threefold-hollow-adsorbed CO and minor linear-adsorbed
CO on Pd NPs, respectively.[40–42] The comparison demon-
strates well that our synthetic method offers the special ability
of heavily loading SA sites on oxide support. Figure S6 (Sup-
porting Information) shows the Pd species anchored in CeO2
without 3DOM structure (Pd/CeO2), synthesized by the in situ

Pd O bonds. The structure parameters of Pd-SAs/3DOM-
CeO2 are further acquired by the least squares EXAFS fitting
(Figure 2b,c) and summarized in Table S3 in the Supporting
Information. In comparison with Pd foil and PdO references,
the fitting results indicate that the Pd atoms in Pd-SAs/3DOM-
CeO2 have a coordination number of about 3 in relation with

the first shell, and the average Pd O bond length is about
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Figure 2. Structure characterizations based on XAFS and DRIFTS. a) FT-EXAFS spectra of Pd-SAs/3DOM-CeO2 in reference to Pd foil and PdO.
b) EXAFS R space fitting curve (red line) and the experimental data (circles) of Pd-SAs/3DOM-CeO2. c) EXAFS k space fitting curve (red line) and the
experimental data (circles) of Pd-SAs/3DOM-CeO2. d) DRIFT spectra of CO molecules absorbed on Pd-SAs/3DOM-CeO2, Pd-SAs/3DOM-CeO2-H2 and
Pd-NPs/3DOM-CeO2, respectively.
pyrolysis process without template. The Pd1.76/CeO2 sample
with a higher Pd loading content about 1.76 wt% presents the
multiple peaks related to the linear-adsorbed CO molecules on
Pd SAs (2151 cm−1) and on Pd clusters or NPs (2107 cm−1) as
well as the hollow-adsorbed CO on Pd NPs (1936 cm−1), respec-
tively. As the Pd loading content is reduced to about 1.0 wt%,
the Pd1.0/CeO2 sample shows only one sharp peak at 2165 cm−1
in its DRIFT spectrum, suggesting the formation of Pd single
atoms on CeO2. In contrast, the 3DOM CeO2 support with
large surface area can provide more sites for anchoring Pd
single atoms, which is beneficial to the anchoring of Pd SAs
with higher content. In addition, when SiO2, a relatively inert
and irreducible oxide, is chosen as an oxide support to syn-
thesize the Pd/3DOM-SiO2 catalyst (Figure S7, Supporting
Information) via the same template-assisted in situ pyrolysis
method, the crystalline PdO is identified to exist in this sample
(Figure S7a, Supporting Information). The CO-probing DRIFT
spectrum (Figure S7c, Supporting Information) shows a broad
infrared (IR) band containing two peaks centered at 2027
and 2004 cm−1, respectively, most likely corresponding to the
bridge-adsorbed CO molecules on Pd2+ and Pd+ species.[43] The
results demonstrate that the 3DOM-CeO2 is a desirable support
to stabilize Pd SAs with a high loading content by the strong
≈335.0 eV is absent, consistent with the fact that Pd atoms are
isolated in Pd-SAs/3DOM-CeO2 instead of forming Pd NPs.
Such a positive shift of Pd XPS signals has been found previ-
ously in the solution-combustion synthesized Pd/CeO2, which
contains Pd in the higher oxidation states being stabilized in
[44]
the Ce3+ sites by a strong interaction of Pd O Ce bonding.
 
The Ce 3d spectra of 3DOM-CeO2 with and without Pd SAs
are shown in Figure 3b. Both of the Ce 3d spectra are deconvo-
luted into eight peaks.[45] The peaks at 884.8 (V’) and 903.2 eV
(U’) correspond to the signals of Ce 3d3/2 and Ce 3d5/2 of Ce3+
species, and the others are assigned to the normal Ce4+ spe-
cies. Interestingly, the peak area ratio of A(Ce(3+)
increases
/Ce( 4 +)
)
after anchoring Pd SAs in 3DOM CeO2, suggesting that the
introduction of Pd SAs promotes the generation of Ce3+ defects
in CeO2. The structures of 3DOM-CeO2 with and without
Pd SAs have also been characterized by Raman spectroscopy
(Figure 3c). The sharp peak at 460 cm−1 belongs to Raman
active F2g mode with fluorite crystal unit. This peak widens and
down-shifts by 4 cm−1 after incorporating Pd SAs into 3DOM-
CeO2, implying that the lattice distortions and the creation of
OVs in CeO2. The peaks at 261, 600, and 1180 cm−1 related to
OVs also become more obvious in the Raman spectrum of Pd-
SAs/3DOM-CeO2. Electron spin resonance (ESR) spectroscopy
(Figure 3d) demonstrates a distinct signal of oxygen vacancy
at the g value of 2.002, confirming the generation of abun-
dant OVs after introducing Pd SAs into 3DOM-CeO2. Taken
together, the XPS, Raman and ESR spectroscopy characteriza-
tions indicate that electron transfer takes place from Pd to Ce4+
through Pd−O−Ce interactions and in turn generates more
Ce3+ defect associated OVs in Pd-SAs/3DOM-CeO2. To reveal
 
Pd O Ce bondings.
The valence states of Pd and Ce associated with their inter-
action in Pd-SAs/3DOM-CeO2 catalyst are analyzed by X-ray
photoelectron spectroscopy (XPS). The Pd 3d5/2 spectrum
(Figure 3a) is fitted with two peaks at 338.2 and 336.5 eV,
which can be assigned to high-valence Pd4+ and Pd2+ species,
respectively.[18,28] The metallic Pd0 with 3d5/2 binding energy at
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Figure 3. Characterization and theoretical simulation of Pd-SAs/3DOM-CeO2. a) Pd 3d XPS spectrum of Pd-SAs/3DOM-CeO2. b) Ce 3d XPS spectra
of 3DOM-CeO2 and Pd-SAs/3DOM-CeO2. c) Roman spectra of 3DOM-CeO2 and Pd-SAs/3DOM-CeO2. d) ESR signals of OVs in 3DOM-CeO2 and Pd-
SAs/3DOM-CeO2. e) Energy evolution and the representative structures during the structural optimization process (Movie S1, Supporting Information)
of Pd-SA/CeO2 model.
the Pd-SA induced formation of surface OVs in CeO2 sup-
port, we employ density functional theory (DFT) calculation to
reveal the structure and energy evolution, as depicted in the
video (Movie S1, Supporting Information) and Figure 3e. The
calculation indicates that introducing Pd atom onto CeO2 (111)
surface causes the spontaneous reconstruction of local struc-
ture, where some oxygen atoms are extracted from CeO2 sur-
face, generating OVs on the surface. In the finally optimized
structure (Figure S8a,b, Supporting Information), Pd atom is
coordinated with three oxygen atoms to form the average Pd−O
distance of 1.84 Å, consistent with the results from EXAFS
measurements. The differential charge density (Figure S8c,
Supporting Information) indicates clearly that the electrons
are transferred from Pd atom to neighboring CeO2, showing
electron depletion at the Pd site. The Mulliken and Hirshfeld
population analyses reveal that the positive charge amount of
each Pd atom is 0.60e and 0.72e, respectively. Taken together,
the strong interaction between Pd atoms and CeO2 support
endows the Pd-SAs/3DOM-CeO2 material with unique merits,
i.e., atomic dispersion and high loading of positively charged
Pd SAs, abundant OVs and mesoporosity in CeO2.
Such unique features of Pd-SAs/3DOM-CeO2 designate it
as an outstanding candidate for catalyzing Suzuki coupling
reactions. After optimizing the catalytic reaction conditions,
including different solvents and base (Table S4, Supporting
Information), the activities of various catalysts are screened
by the coupling reaction of iodobenzene and phenylboronic
acid in the optimal system, which is performed directly under
ambient conditions (298 K, air atmosphere). The results
are summarized in Figure 4a and Table S5 (Supporting
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Figure 4. Catalytic performance toward the Suzuki reaction of iodobenzene and phenylboronic acid. a) Percent conversion of iodobenzene and selec-
tivity for producing biphenyl over different catalysts. b) Arrhenius plots of the reaction over three different catalysts. c) Durability tests of Pd-SAs/3DOM-
CeO2 for the Suzuki reaction (4 h for each cycle). d) Pd 3d XPS spectrum of Pd-SAs/3DOM-CeO2 after eight cycles.
Information). The reaction cannot occur in the absence of Pd,
indicating the catalytic inactivity of CeO2 support. Notably, the
Pd-SAs/3DOM-CeO2 catalyst exhibits excellent activity and
selectivity (100%) toward the Suzuki coupling reaction. The
iodobenzene conversion presents a turnover frequency (TOF)
value of 750.99 h−1 at the low conversion level (Table S7, Sup-
porting Information) and achieves a high percent conversion
of 91.50% after 4 h. In contrast, the homogeneous Pd(PPh3)4
catalyst shows quite poor performance under either air or N2
atmosphere. Moreover, the activity of Pd-SAs/3DOM-CeO2
catalyst is much higher than that of the commercial Pd/C
catalyst with 5 wt% Pd loading (Figure S9, Supporting Infor-
mation). These manifest the superiority of this SA catalyst to
the homogeneous and traditional heterogeneous catalysts in
Suzuki coupling reactions.
of Pd-SA active sites. Moreover, the Pd/3DOM-SiO2 catalyst
also performs a poor activity, with iodobenzene conversion
of 53.26%, confirming the pivotal role of the 3DOM-CeO2
support.
The influence of Pd content in Pd-SAs/3DOM-CeO2 on the
catalytic activity is further studied (Table S6 and Figure S10a,
Supporting Information). The percent conversion of iodoben-
zene is boosted from 55.41% to 91.50% after the same reaction
time of 4 h as the Pd content increases from 0.61 to 1.76 wt%,
whereas the conversion drops down to 75.52% when the Pd
content further increases to 2.36 wt%. The increased Pd con-
tent can afford more active sites in the Pd-SAs/3DOM-CeO2
catalyst. However, too heavy Pd loading leads to the formation
of Pd clusters (Figure S10b, Supporting Information) which
reduces the efficient active sites.
To demonstrate the advantages of atomically dispersing Pd
active sites and the 3DOM-CeO2 support, we also examine the
catalytic performance of other as-prepared catalysts. Specifi-
cally, the Pd-SAs/3DOM-CeO2-H2 catalyst presents a reduced
activity, compared with Pd-SAs/3DOM-CeO2, acquiring the
iodobenzene conversion of 68.28% after 4 h. Pd-NPs/3DOM-
CeO2 exhibits a further depressed iodobenzene conversion of
48.62%. The Pd1.76/CeO2 sample synthesized through the in
situ pyrolysis process without template shows the iodoben-
zene conversion of 67.47% after 4 h, due to the non-porous
structure of CeO2 support as well as the formation of Pd clus-
ters and NPs. Although the Pd1.0/CeO2 sample has a reduced
Pd loading content, compared with the Pd1.76/CeO2 sample,
it exhibits increased activity with the iodobenzene conver-
sion of 73.34% after 4 h, which further proves the superiority
To acquire the reaction kinetics over several different cata-
lysts, we further take the coupling reaction between iodo-
benzene and phenylboronic acid in the temperature range of
293–313 K. The Suzuki coupling reactions on Pd-SAs/3DOM-
CeO2 can be accelerated substantially by slightly elevating the
temperature. As a result, the iodobenzene conversion (%)
versus reaction time (min) curves at different temperatures
(293, 298, 303, 308, and 313 K) can be obtained (Figure S11a–c,
Supporting Information). The relationship of ln(C0/Ct) versus
time turns out to match well with the first-order reaction
kinetics (Figure S11d–f, Supporting Information). After cal-
culating the corresponding reaction rate constants (k) by the
slope of ln(C0/Ct)-time curves, the Arrhenius plots are sub-
sequently obtained by drawing lnk as a function of 1000/T
(Figure 4b). The reaction on Pd-SAs/3DOM-CeO2 has a lower
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apparent activation energy of 0.95 eV than the ones catalyzed
by Pd-SAs/3DOM-CeO2-H2 (1.10 eV) and Pd-NPs/3DOM-CeO2
(1.29 eV), respectively. This suggests that the integration of
Pd-SA sites with 3DOM CeO2 support can effectively reduce
the reaction activation energy and thus boost the coupling
reactions.
Recyclability is another key parameter for evaluating the
performance of catalysts in addition to activity and selectivity.
The recycling experiments of Pd-SAs/3DOM-CeO2 catalyst
have been performed at the low conversion (1 h for each cycle,
Figure S12, Supporting Information) and the high conversion
(4 h for each cycle, Figure 4c), respectively. In both the cases,
high activity and selectivity of Pd-SAs/3DOM-CeO2 catalyst
are well maintained after eight reaction cycles. XRD, XPS, and
TEM characterizations (Figure 4d and Figure S13, Supporting
Information) indicate that the chemical compositions and
3DOM structure of the Pd-SAs/3DOM-CeO2 catalyst are well
maintained after eight catalytic cycles, excluding the agglom-
eration of Pd atoms. HAADF-STEM image with the intensity
profiles and CO-probing DRIFT spectrum (Figure S14, Sup-
porting Information) further confirm that the Pd-SAs/3DOM-
CeO2 catalyst retains the feature of Pd SAs after recycling.
Moreover, it has been excluded that the possibly leached Pd
species in solution has acted as a homogeneous catalyst, by
monitoring the coupling reaction in the filtrate (Figure S15,
Supporting Information). After a reaction cycle catalyzed by
Pd-SAs/3DOM-CeO2, the catalyst particles are removed from
the solution by high-speed centrifugation and filtration. Then
the reactants of iodobenzene, phenylboronic acid and K2CO3
are added into the filtrate for reaction, where no more biphenyl
can be produced even after 4 h. The Pd concentration of the
filtration (10 mL) is measured to be 0.626 ppb by ICP-MS, indi-
cating that only a negligible fraction of Pd total mass (0.012%)
in the catalyst has entered into the solution. The information
gleaned above proves that the Pd-SAs/3DOM-CeO2 is a highly
stable catalyst and the Pd SAs are not detached from CeO2
support or aggregate into NPs during the long-term reaction
process.
To get more insights into the high activity of Pd-SAs/3DOM-
CeO2 catalyst, the reaction pathways and corresponding
energy profiles of Suzuki coupling reaction occurring on the
Pd-SA/CeO2 catalyst model are simulated by DFT calcula-
tions (Figure 5). Suzuki coupling reaction proceeds generally
through a three-step pathway: oxidative addition (dissociative
adsorption of arylhalide), transmetalation (dissociative adsorp-
tion of alkali-activated phenylboronic acid), and reductive
elimination (coupling of two phenyl groups).[3,46] As shown in
Figure 5, the oxidative addition of iodobenzene to Pd SA needs
to overcome an energy barrier of 1.06 eV (TS1). However, there
is only a slight barrier (0.04 eV) in the transmetalation step
(TS2), indicating that this step can be easily achieved on Pd SA
site. As a final step, the coupling between two phenyl groups
on a Pd atom is barrier-less and highly exothermic. Therefore,
the rate-determining step of this pathway is the oxidative addi-
tion step with an activation barrier of 1.06 eV, which agrees well
with the experimental value (0.95 eV) from the Arrhenius plots
of this reaction on Pd-SAs/3DOM-CeO2 catalyst. In sharp con-
trast, the Pd-cluster/CeO2 catalyst model exhibits a different
rate-determining step for the Suzuki reaction (Figure S16,
Figure 5. DFT calculated reaction pathway and the corresponding energy
profiles of Suzuki reaction between iodobenzene and phenylboronic acid
on Pd-SA/CeO2 catalyst model. The gray, white, purple, pink, red, yellow,
and blue spheres represent carbon, hydrogen, iodine, boron, oxygen,
cerium, and palladium, respectively.
Supporting Information). In addition to a barrier of 1.44 eV
existing in the transmetalation step for B−C dissociation, the
coupling of two phenyl groups (reductive elimination step) on
Pd cluster has to overcome a higher barrier of 2.26 eV. The
experimental activation energies of this Suzuki reaction on
Pd-SAs/3DOM-CeO2-H2 (1.10 eV) and Pd-NPs/3DOM-CeO2
(1.29 eV) are much lower than the DFT calculated barrier of the
rate-determining step on Pd-cluster/CeO2 catalyst model, most
likely because the actual catalysts contain some Pd SA sites that
contribute to lowering the overall activation energies. Never-
theless, the DFT calculation has unambiguously indicated that
the Pd-SA/CeO2 catalyst can remarkably reduce the barriers of
transmetalation and reductive elimination steps of the Suzuki
reaction.
The IR spectra of phenylboronic acid adsorbed on dif-
ferent catalysts (Figure S17, Supporting Information) vali-
date experimentally that the Pd-SAs/3DOM-CeO2 catalyst
is more competent to activate phenylboronic acid and pro-

mote the cleavage of B C bond, thus greatly lowering the
energy barrier of transmetalation step. The B−C stretching
[47,48]
vibration (≈1050 cm−1)
of phenylboronic acid adsorbed
on Pd-SAs/3DOM-CeO2 shifts (10.6 cm−1) more toward low
frequency region than those on Pd-SAs/3DOM-CeO2-H2
(9.7 cm−1) and Pd-NPs/3DOM-CeO2 (8.7 cm−1), suggesting

the more activated B C bond on Pd-SAs/3DOM-CeO2 cata-
lyst. As a matter of fact, the electronic states of active centers
play a key role in altering their catalytic activities toward the
Suzuki coupling reaction, because the catalyst is required to
serve as a charge donor in the oxidative addition step and a
charge acceptor in both transmetalation and reductive elimi-
nation steps of the reaction.[49–51] The Pd SA sites in our Pd-
SAs/3DOM-CeO2 catalyst are positively charged owing to the
electron transfer from Pd atoms to the neighboring CeO2
units, which has been verified by the aforementioned experi-
mental and theoretical results. The high-valence Pd4+ sites in
Pd-SAs/3DOM-CeO2 catalyst can serve as a strong electron
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acceptor to facilitate both the electron-withdrawing steps of
transmetalation and reductive elimination. Meanwhile, the
3DOM-CeO2 support containing abundant OVs not only
affords the adaptive coordination of Pd SAs, but also modi-
fies their electronic structures. The Ce3+ species associated
with OVs may donate electrons to the high-valence Pd4+ sites
and thus improve the charge-donating ability of the latter
for boosting the oxidative addition step to some extent. In
contrast, the Pd cluster is more apt at charge donating than
electron accepting, leading to the great difficulties in imple-
menting transmetalation and reductive elimination steps. In
terms of geometric effects, the two phenyl groups bonded on
a single Pd atom are very close to each other, greatly facili-
tating the C–C coupling to produce biphenyl via the reduc-
tive elimination step. On Pd cluster, however, the two phenyl
groups have to migrate to a Pd atom for C–C coupling, which
further increases the energy barrier of the reductive elimi-
nation step. In brief, the Pd SAs anchored in 3DOM-CeO2
support can act as highly active sites for the Suzuki reactions
to remarkably reduce the barriers of both transmetalation
and reductive elimination steps, owing to their atomically
dispersive and positively charged traits. These traits can be
well maintained in long-term catalytic reactions because
of the strong metal-support interaction through the unique
The Pd-SAs/3DOM-CeO2 catalyst also exhibits excellent per-
formance toward the cross coupling reactions involving het-
eroaromatics, such as 4-pyridylboronic acid, 2-iodopyridine
and 2-bromopyridine (Table 1, entries 13–15). As the tempera-
ture is raised a little to 303 K, the percent conversions of bro-
mobenzene and its derivatives can rise to around 98% (Table 1,
entries 16–18) after 4 h. With further elevating the temperature
to 348 K, the percent conversion of chlorobenzene increases
to 64.40% after 4 h (Table 1, entry 19). As increasing the sub-
strate concentrations at the elevated temperature of 348 K,
the TOF value for bromobenzene conversion can reach up to
44 527.10 h−1 (Table S7 (Supporting Information), entry 20).
Overall, the performance of Pd-SAs/3DOM-CeO2 catalyst not
only far exceeds that of the homogeneous Pd(PPh3)4 catalyst
but also much superior to those of Pd-SAs/3DOM-CeO2-H2,
Pd-NPs/3DOM-CeO2 and commercial Pd/C with 5 wt% Pd in
all the listed Suzuki coupling reactions (Figures S20 and S9,
Supporting Information).
3. Conclusion
In summary, the heterogeneous single-atom catalyst of Pd-
SAs/3DOM-CeO2 has been synthesized successfully by a tem-
plate-assisted in situ pyrolysis method. This delicate design and
convenient synthesis of material perfectly take advantage of the
strong interactions between Pd SAs and 3DOM-CeO2 support,
enabling a set of important characteristics in the final material.
The high surface area of 3DOM-CeO2 support facilitates the
anchoring of Pd SAs with a high content of 1.76 wt%, while
the presence of Pd SAs induces the generation of abundant
OVs and prevents the grain growth of CeO2 under the high-
temperature treatment. Benefiting from the unique structural
and electronic characteristics, the Pd-SAs/3DOM-CeO2 catalyst
exhibits outstanding activity and durability toward Suzuki reac-
tions for a broad scope of substrates under ambient conditions,
far outperforming the homogeneous Pd(PPh3)4 catalyst and
the other listed heterogeneous catalysts. The TOF values for
the conversions of iodo-, bromo- and chlorobenzene under the
ambient conditions are 750.99, 520.01, and 163.26 h−1, respec-
tively. Elevating the temperature to 348 K, TOF of bromoben-
zene conversion can reach up to 44 527.10 h−1. Theoretical
calculations have revealed that the CeO2 supported Pd SAs can
remarkably reduce the energy barrier of transmetalation and
reductive elimination steps for the Suzuki reaction, showing a
lower activation energy of the cross-coupling reaction. During
the long-term reaction, Pd SAs can be stabilized on CeO2 by
 
Pd O Ce bonds. As a result, the Pd-SAs/3DOM-CeO2 cata-
lyst shows excellent activity and stability.
The universal application of the Pd-SAs/3DOM-CeO2 cata-
lyst is demonstrated by the Suzuki coupling reactions of var-
ious substrates. This catalyst exhibits excellent activity toward
a broad scope of substrates (Table 1), far outperforming the
homogeneous Pd(PPh3)4 catalyst. Moreover, the TOF values
for bromobenzene and chlorobenzene conversion under the
ambient conditions are 520.01 and 163.26 h−1 (Table S7, Sup-
porting Information), respectively, which are appealing results
under such mild conditions compared with the previous
reports (Table S8, Supporting Information). The percent con-
versions of bromobenzene and chlorobenzene can also achieve
the considerable levels of 76.61% and 37.62% (Table 1, entries
2–3) after 4 h under the ambient conditions. The isolation yield
of the coupling reaction between bromobenzene and phenylb-
oronic acid over Pd-SAs/3DOM-CeO2 catalyst is analyzed as a
typical example (Figure S18, Supporting Information). After a
series of purification procedure, the isolation yield is obtained
to be 70.69%, which is a little lower than that determined by gas
chromatography (GC) analysis due to the product loss caused
by the purification process. The conversion of chlorobenzene
cannot be further obviously improved, until the temperature
is raised to 348 K (Figure S19, Supporting Information), pos-
sibly because the coordination of Cl− ions to Pd atoms leads to
the poisoning of the active sites in Pd-SAs/3DOM-CeO2 cata-
lyst.[52] The arylhalides with electron-donating groups (Table 1,
entries 5–9 and 18) show slightly reduced conversion rates,
whereas the ones with electron-withdrawing groups (Table 1,
entries 4 and 17) exhibit the increased reactivities. With the
same substituent groups (entries 7–9), the conversion rate of
ortho-substituted arylhalide is relatively slower than those
of the para- and meta- ones due to the steric effect. The types
of the substituents on phenylboronic acid have no obvious
effect on the conversions of arylhalides (Table 1, entries 10–12).
 
the strong Pd O Ce bonds, preventing the leaching or aggre-
gation of Pd atoms. Therefore, the 3DOM CeO2 is a desirable
support to serve as the solid-state ligand to Pd SAs as well as
a charge modulator for tailoring the electronic states of Pd
catalytic sites. This research not only demonstrates the facile
synthesis of Pd SAs catalyst with well-designed support but
also offers insights into the regulation mechanism of catalytic
sites achieved by the strong metal−support interactions. Such
Pd SAs catalysts possess the advantages of both homogeneous
and heterogeneous catalysts in organic transformations and
are expected to develop a new level for green synthesis of fine
chemicals in the future.
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Table 1. Catalytic activities of Pd-SAs/3DOM-CeO2 catalyst toward some typical Suzuki reactions of different substrates, with homogeneous
Pd(PPh3)4 catalyst for comparison. Entries 1–15: aryl halide (0.5 mmol), phenylboronic acid (0.6 mmol), K2CO3 (1.5 mmol), catalyst (3 mg), EtOH/
H2O (2 mL/2 mL), 298 K and air atmosphere, 4 h. Entries 16–18: the same conditions as entries 1–15 except that the temperature is 303 K. Entry 19:
the same conditions as entries 1–15 except that the temperature is 348 K.
Entry
1
Aryl halide
Phenylboronic acid
Yield (%)a)
91.50
Yield (%)b)
2.54 (20.87c))
2
3
4
5
6
7
8
76.61
37.62
94.36
85.17
85.74
89.37
85.66
1.87 (12.25c))
1.52 (3.66c))
3.24
2.43
2.35
2.41
2.29
9
82.51
2.27
10
11
12
13
14
90.33
93.21
91.37
90.19
92.55
1.83
1.24
2.58
2.38
3.48
15
89.06
2.39
16
17
98.10
98.68
8.87
10.78
18
19
96.10
64.40
7.59
14.31
a)Pd-SAs/3DOM-CeO2; b)Pd(PPh3)4; c)N2 atmosphere. All the reactions are equally (100%) selective to the desired cross-coupled product.
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Acknowledgements
X.T. and R.L. contributed equally to this work. This work was financially
supported by the National Natural Science Foundation of China
(21471004, 61775129, 21725102, 91961106, U1832156, 21601173), the
National Key R&D Program of China (2017YFA0207301), the Natural
Science Foundation of Zhejiang Province of China (No. LY19B010005),
the Fundamental Research Funds of Zhejiang Sci-Tech University
(2020Y003) and the Fundamental Research Funds for the Central
Universities (GK201902001). The authors thank the National Synchrotron
Radiation Laboratory (Hefei, China) for DRIFTS measurement (BL01B),
the Shanghai Synchrotron Radiation Facility (SSRF, China) for EXAFS
measurement (BL14W1), Dr. Shoujie Liu from Anhui Normal University
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Conflict of Interest
The authors declare no conflict of interest.
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