A Novel Approach to High-Performance Aliovalent-Substituted Catalysts—2D Bimetallic MOF-Derived CeCuOx Microsheets

Article abstract of DOI:10.1002/smll.201903525

Mixed transition metal oxides (MTMOs) have enormous potential applications in energy and environment. Their use as catalysts for the treatment of environmental pollution requires further enhancement in activity and stability. This work presents a new synthesis approach that is both convenient and effective in preparing binary metal oxide catalysts (CeCuO_x) with excellent activity by achieving molecular-level mixing to promote aliovalent substitution. It also allows a single, pure MTMO to be prepared for enhanced stability under reaction by using a bimetallic metal–organic framework (MOF) as the catalyst precursor. This approach also enables the direct manipulation of the shape and form of the MTMO catalyst by controlling the crystallization and growth of the MOF precursor. A 2D CeCuO_x catalyst is investigated for the oxidation reactions of methanol, acetone, toluene, and o-xylene. The catalyst can catalyze the complete reactions of these molecules into CO₂ at temperatures below 200 °C, representing a significant improvement in performance. Furthermore, the catalyst can tolerate high moisture content without deactivation.

Full text of DOI:10.1002/smll.201903525

COMMUNICATION
Bimetallic Catalysts
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A Novel Approach to High-Performance
Aliovalent-Substituted Catalysts—2D Bimetallic
MOF-Derived CeCuOx Microsheets
Qingyue Wang, Zhimin Li, Miguel A. Bañares, Lu-Tao Weng, Qinfen Gu, Jason Price,
Wei Han,* and King Lun Yeung*
interest in mixed transition metal oxides
(MTMOs). Our recent study[6] and those of
others[7,8] have shown that surface defects
of catalysts, particularly surface oxygen
vacancies, play an important role in cata-
lytic performance. Aliovalent substitution
is considered to promote the creation of
defects and vacancies, thus enhancing
the catalytic reaction.[9,10] Conventional
preparation methods using wet impregna-
tion,[11,12] combustion,[13] and co-precipi-
tation[14] are unable to produce uniform
substitution beyond 10 at.%.[15] Higher
loading often triggers the formation of
separate oxide phases. Moreover, high-
temperature post-treatment often results
in sintering and loss of surface area for
MTMOs prepared by these methods.
Mixed transition metal oxides (MTMOs) have enormous potential applica-
tions in energy and environment. Their use as catalysts for the treatment of
environmental pollution requires further enhancement in activity and stability.
This work presents a new synthesis approach that is both convenient and
effective in preparing binary metal oxide catalysts (CeCuOx) with excellent
activity by achieving molecular-level mixing to promote aliovalent substitu-
tion. It also allows a single, pure MTMO to be prepared for enhanced stability
under reaction by using a bimetallic metal–organic framework (MOF) as the
catalyst precursor. This approach also enables the direct manipulation
of the shape and form of the MTMO catalyst by controlling the crystalliza-
tion and growth of the MOF precursor. A 2D CeCuOx catalyst is investigated
for the oxidation reactions of methanol, acetone, toluene, and o-xylene. The
catalyst can catalyze the complete reactions of these molecules into CO2 at
temperatures below 200 °C, representing a significant improvement in perfor-
mance. Furthermore, the catalyst can tolerate high moisture content without
deactivation.
Metal–organic frameworks (MOFs) were
first used as catalyst precursor in making
electrocatalysts.[16–18] More recently, ZIF-67
was used to prepare CeO2-doped Co3O4 to
Transition metal oxides are used in many technological prod-
ucts in the energy and environmental fields including cata-
lysts,[1,2] sensors,[3,4] and electrodes.[5] Being less expensive than
noble metals, transition metal oxides are attractive as environ-
mental catalysts, but reactivity and selectivity remain major
hurdles. Studies show that doping and lattice substitution
can enhance the catalytic activities, thus explains the growing
catalyze the reaction of NO and CO.[19] Zamaro et al.[20] pyro-
lyzed the wet-impregnated Ce(III) on HKUST-1 to obtain CuO/
CeO2 catalyst for CO oxidation. Similarly, Li and co-workers[21]
calcined Cu2+ supported on Ce-based UiO-66 to obtain CuO/
CeO2 active for CO oxidation at around 100 °C. The degree of
heterometal substitution into metal oxide lattices is limited for
these examples. This work proposes a novel synthesis approach
Dr. Q. Wang, Dr. Z. Li, Prof. L.-T. Weng, Prof. K. L. Yeung
Department of Chemical and Biological Engineering
The Hong Kong University of Science and Technology
Clear Water Bay, Kowloon, Hong Kong SAR, China
E-mail: kekyeung@ust.hk
Prof. L.-T. Weng
Materials Characterization & Preparation Facility
The Hong Kong University of Science and Technology
Clear Water Bay, Kowloon, Hong Kong SAR, China
Dr. Q. Gu, Dr. J. Price
Dr. Z. Li
Australian Synchrotron (ANSTO)
800 Blackburn Road, Clayton, VIC 3168, Australia
School of Mechatronical Engineering
Beijing Institute of Technology
5 South Zhongguancun Street 100081, Beijing, China
Dr. W. Han, Prof. K. L. Yeung
Division of Environment and Sustainability
The Hong Kong University of Science and Technology
Clear Water Bay, Kowloon, Hong Kong SAR, China
E-mail: keweihan@ust.hk
Prof. M. A. Bañares
Spectroscopy and Industrial Catalysis Group
Instituto de Catálisis y Petroleoquímica
ICP-CSIC
Dr. W. Han, Prof. K. L. Yeung
Guangzhou HKUST Fok Ying Tung Research Institute
S&T Building, Nansha IT Park
Marie Curie 2, E-28049 Madrid, Spain
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.201903525.
Nansha District 511458, Guangzhou, China
DOI: 10.1002/smll.201903525
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Scheme 1. Schematic diagram of the synthesis procedure of bimetallic MOF-derived CeCuOx catalyst.
by first preparing a pure bimetallic MOF from target metals
(e.g., copper and cerium in this study), followed by careful
MOF transformation into MTMOs to achieve a high level of
aliovalent substitution without triggering phase separation. In
comparison to conventional syntheses[22–24] or single metallic
MOF-templated approaches,[20,21] this method achieves mole-
cular-level mixing at the MOF precursor level, thus promoting
an aliovalent substitution process to attain high oxygen storage
capacity, good redox property, and reoxidation ability.[25]
crystal has sides of 4 1 µm and a thickness of 0.25 0.05 µm.
The transmission electron microscopy (TEM) image confirms
the layered structure of the bimetallic MOF (Figure 1b). The
high-resolution image in Figure 1c shows that the distance
between neighboring metal clusters is about 1.1 nm, which is
consistent with the results obtained by SXRD for the distance
between neighboring clusters along the (011) planes.
The elemental compositions of the bimetallic MOFs were
determined by energy-dispersive X-ray (EDX) spectroscopy
(Oxford Instrument X-Max). The sample was deposited on a
polished silicon wafer. It can be seen from Figure 2a that the
elemental mapping images of cerium and copper are uniform
over the entire crystal, indicating homogeneous compositions.
The carbon and oxygen signals are from the organic ligands.
Thermal decomposition of the bimetallic MOF, as illustrated in
Scheme 1, leads to the removal of organic ligands and forma-
tion of CeCuOx with lattice vacancies and strains from aliova-
lent substitution of copper into ceria. Figure 2b shows the EDX
elemental mapping images of CeCuOx. It can be seen that the
elemental mapping images of cerium and copper remained uni-
form and relatively unchanged when compared to the pristine
MOF. However, the carbon signal disappeared, indicating the
complete removal of organic ligands and the transformation
from bimetallic MOF to MTMOs. High-resolution transmission
electron microscopy (HRTEM) images reveal the formation of a
mesoporous solid (Figure 2c) that consists of interpenetrating
CeO2-rich and CuO-rich domains shown in Figure 2d. The areas
are outlined in dashed lines at the grain boundaries between
impinging CeO2-rich and CuO-rich domains (Figure 2d) that
display crystal lattice strains (Figure 2e) and crystal defects
(Figure 2f). These defects are mainly oxygen vacancies that are
crucial to the catalytic performance of ceria catalysts for com-
plete oxidation of volatile organic compounds (VOCs).[26,27]
The X-ray diffraction pattern of CeCuBDC bimetallic MOF
is unique and different from those of CeBDC and CuBDC
MOFs. The bimetallic MOF is a monoclinic crystal with P21/c
The bimetallic MOFs were synthesized via a solvothermal
method by accurately manipulating solvent/water ratio, and
the MTMOs were further prepared by the thermal decomposi-
tion of bimetallic MOFs. The synthesis procedure, as illustrated
in Scheme 1, involves solutions of N,N-dimethylformamide
(DMF) containing the metal cations (i.e., Ce3+ and Cu2+) and the
organic ligands (i.e., dianions derived from 1,4-benzenedicarbo-
xylic acid (H2BDC)). The two solutions were mixed in a Teflon-
lined stainless steel autoclave and heated to 80 °C for 24 h. The
CeCuBDC bimetallic MOF was recovered by centrifugation
and washing steps described in the Supporting Information.
The bimetallic MOF has a monoclinic crystal structure (space
group P21/c) with a unit cell of a = 12.110(2) Å, b = 18.490(4) Å,
c = 12.029(2) Å, β = 114.13(3)°. Single crystal X-ray diffraction
(SXRD) measurement was carried out using the macromo-
lecular crystallography (MX) beamline (Australian Synchrotron,
ANSTO). The structure of CeCuBDC was solved and refined by
using SHELXTL software. Each unit cell of CeCuBDC consists
of four Ce3+ ions, two Cu2+ ions, and eight 1,4-benzenedicar-
boxylic (BDC) dianions. Bimetallic MOF (i.e., CeCuBDC with
molecular mixing of two metal clusters) was further thermal-
transformed into CeCuOx to create crystal strains and abundant
defects at the grain boundaries for high catalytic performances,
as sketched in Scheme 1.
The CeCuBDC bimetallic MOF forms a 2D layered struc-
ture as shown by the scanning electron microscopy (SEM)
image obtained under a JEOL JSM 7100F microscope at an
accelerating voltage of 15 kV (Figure 1a). The 2D rhombic
space group, while CeBDC has a triclinic crystal structure
P1
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Figure 1. a) SEM, b) TEM, and c) HRTEM images of CeCuBDC MOF and (inset) schematic drawing of the unit cell. (Note: The yellow dashed lines
indicate the distribution of metal ions and organic ligands along (011) plane.)
Figure 2. SEM–EDX elemental mapping images of a) CeCuBDC and b) CeCuOx prepared via thermal transformation of CeCuBDC, along with the
HRTEM images of CeCuOx showing the formation of c) mesopores, d) Ce-rich and Cu-rich domains, e) crystal strains, and f) vacancies.
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Figure 3. a) X-ray diffraction patterns, b) thermogravimetric profiles of MOF precursors, and c) X-ray diffraction patterns and d) micro-Raman spectra
of CeO2, CuO, and CeCuOx derived from the corresponding MOFs (i.e., CeBDC, CuBDC, and CeCuBDC).
with a unit cell (a = 11.069(2) Å, b = 17.744(4) Å, c = 29.089(6) Å,
α = 87.13(3)°, β = 79.38(3)°, and γ = 72.99(3)°) as shown in
Figure 3a. CuBDC also has a monoclinic crystal structure, but a
space group of C2/m and unit cell parameters of a = 11.414 Å,
b = 14.269 Å, c = 7.780 Å, β = 108.12°.[28] The preferential
growth of the (011) planes leads to CeCuBDC bimetallic MOF
with 2D morphology. Figure 3b plots the thermogravimetric
analysis results of the CeBDC, CuBDC, and CeCuBDC MOFs
in dry synthetic air at a heating rate of 10 °C min−1. It can be
seen that the thermal decomposition of CeCuBDC occurs at
329 °C, while decomposition temperatures of CeBDC and
CuBDC are 379 and 326 °C, respectively. CeCuBDC loses 68%
of its original weight, which is consistent with theoretical cal-
culations for the weight loss from Ce2Cu(BDC)4(DMF)4·6H2O
transformation to Ce2CuO5. According to thermogravimetric
analysis results, here we prepared CeCuOx, CeO2, and CuO
by calcining CeCuBDC, CeBDC, and CuBDC at 350, 400, and
350 °C, respectively.
The powder X-ray diffraction (PXRD) patterns were obtained
from PANalytical X’pert diffractometer with a 2 kW CuKα
X-ray source and a graphite monochromator. It can be seen in
Figure 3c that the diffraction peaks of CeCuOx are shifted by
0.3° compared to those ascribed to (111), (200), (220), and (331)
planes of CeO2. This shift means that CeCuOx has a smaller lat-
tice parameter than CeO2 (Table S5, Supporting Information),
strongly indicating an aliovalent substitution of copper into ceria
lattice.[23,29] The crystal sizes of the catalysts calculated from the
diffraction peak broadening using the Debye–Scherrer equation
(Tables S6 and S7, Supporting Information) are consistent with
HRTEM results (Figure 2; Figure S14, Supporting Informa-
tion). From Table S6 and S7 (Supporting Information), CeCuOx
has the crystallite size of 6.6 nm, while the crystallite sizes of
CeO2 and CuO are 8.5 and 31.7 nm, respectively. The Raman
spectrum of CeCuOx (Figure 3d) displays F2g band of CeO2 at
1
449 cm−1 and Ag and Bg bands of CuO at 280 and 316 cm−1.
A redshift of 15 cm−1 for F2g band of CeO2 and a blueshift of
10 cm−1 for Ag band of CuO observed in the spectrum of CeCuOx
are further evidence of copper substitution and possible forma-
tion of Ce3+ (or Cu+).[30–32] The broadening of F2g band of CeO2
is attributed to the oxygen vacancies.[33] The CeO2 defect-related
vibration (around 600 cm−1) on CeCuOx spectrum overlapped
2
with CuO Bg mode at 603 cm−1 and cannot be clearly meas-
ured. Nitrogen physisorption isotherms shown in Figure S11
(Supporting Information) indicate that all three catalysts are
mesoporous materials. Their specific surface area and pore struc-
tural data are summarized in Table S7 (Supporting Information).
The pore size distribution (Figure S11, Supporting Information)
calculated from adsorption branch based on Barrett–Joyner–
Halenda (BJH) model[34] shows that the average pore size of
CeCuOx is 4.6 nm, which is consistent with the measurement
result of HRTEM image (Figure 2c).
Figure 4 presents full X-ray photoelectron spectroscopy (XPS)
spectra along with O 1s, Ce 3d, and Cu 2p narrow scan spectra
of CeO2, CuO, and CeCuOx. Both C 1s and N 1s signals were
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Figure 4. a) Full XPS spectra, b) O 1s, c) Ce 3d, and d) Cu 2p narrow scan XPS spectra of CeO2, CuO, and CeCuOx.
significantly lower with the removal of the organic ligand and
solvent molecules during the thermal transformation from
CeCuBDC bimetallic MOF to CeCuOx MTMOs (Figure S12 and
Table S8, Supporting Information). No significant enrichment
of cerium or copper was observed on the surface of CeCuOx
after thermal transformation of CeCuBDC (Table S8, Supporting
Information). Cerium exists as Ce3+ in bimetallic MOF (Figure
S12b, Supporting Information), while Ce species is mainly Ce4+ in
CeCuOx (Figure 4c). The Ce3+ is identified by Ce 3d3/2 bands at u0
(900.3 eV) and u′ (902.2 eV) and Ce 3d5/2 bands at v0 (880.2 eV)
and v′ (884.3 eV). Meanwhile, Ce4+ is ascribed by Ce 3d3/2 bands
of u (901.2 eV), u′′ (907.9 eV), u′′′ (917.0 eV), and Ce 3d5/2 bands
of v (882.5 eV), v′′ (889.3 eV), v′′′ (898.6 eV).[35,36] Compared to
CeO2, CeCuOx has higher Ce3+ content (Table 1). Roughly 14%
of the copper in CeCuOx exists as Cu+ determined by Cu 2p3/2
band at 932.4 eV, and the remaining as Cu2+ corresponding to
Cu 2p3/2 band at 933.6 eV (Figure 4d).[37,38] The shake-up sat-
ellites also indicate the existence of Cu2+.[37] The O 1s spectra
show that in addition to the lattice oxygen Oα (529.6 eV), there
is a separate band Oβ (531.5 eV) attributed to adsorbed oxygen
species adjacent to oxygen vacancies (Figure 4b).[8,29,39] Table 1
shows that the Ce3+/(Ce4++Ce3+) ratio of CeCuOx (26.2%) is
larger than that of CeO2 (23.1%), and this is also true for Cu+/
(Cu2++Cu+) of CeCuOx (14.4%) compared to CuO (11.1%).
CeCuOx also has higher surface oxygen vacancy concentration
with the Oβ/(Oα+Oβ) ratio of 33.3% compared to CeO2 (29.4%)
and CuO (27.1%). The interaction between cerium and copper
in CeCuOx results in shifts of −0.3 eV for Ce 3d and +0.4 eV
for Cu 2p. These significant shifts in binding energies confirm
the aliovalent substitution of copper into ceria lattice within
CeCuOx.
The prepared bimetallic oxide CeCuOx was used as the oxi-
dation catalyst of organic compounds, including methanol,
Table 1. Summaries of surface oxygen vacancy, Ce3+, and Cu+ on CeO2,
CuO, CeCuOx, and CeCuBDC and activation energy (Ea) for the toluene
oxidation reaction.
Oβ/(Oα+Oβ) Ce3+/(Ce4+ + Ce3+
) ]
Cu+/(Cu2+ + Cu+) Ea [kJ mol−1
Samples
CeO2
29.4%
27.1%
33.3%
4.7%
23.1%
–
–
57.5
70.5
52.4
–
CuO
11.1%
14.4%
10.9%
CeCuOx
CeCuBDC
26.2%
86.1%
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Figure 5. a) Toluene conversion and CO2 yield versus temperature curves over CeO2, CuO, and CeCuOx catalysts. b) Conversion versus temperature
curves for complete oxidation of methanol, acetone, toluene, and o-xylene over CeCuOx catalyst. (Note: [VOC] = 500 ppm, mcatalyst = 0.15 g, gas hourly
space velocity (GHSV) = 23 000 h−1
)
acetone, toluene, and o-xylene that are common volatile organic
pollutants in air. The CeO2 and CuO catalysts described in
Figures 3c,d and 4 were used for comparison. Figure 5a plots
the toluene conversion and CO2 yield at different reaction tem-
peratures. It is clearly observed that CeCuOx is more active than
both CeO2 and CuO catalysts. Table 1 shows that CeCuOx has
the lowest activation energy of 52.4 kJ mol−1 for toluene oxida-
tion compared to CeO2 (57.5 kJ mol−1) and CuO (70.5 kJ mol−1),
which explains the lowest light-off temperature of CeCuOx cata-
lyst. Two parameters, the temperatures required to attain 50%
(T50) and 90% (T90) conversions, were used to evaluate catalytic
activities for VOC oxidation. The CeCuOx catalyst exhibits the
best performance with T50 and T90 of 150 and 186 °C, respec-
tively. The CeO2 catalyst requires a temperature in excess of
300 °C to attain complete conversion of toluene to CO2. The
high light-off temperature of CuO indicates that it has poor
reactivity, which results from its low oxygen storage capacity
and limited surface area (≈20.4 m2 g−1; Table S7, Supporting
Information). The CeCuOx catalyst displays a significant
improvement over literature reports including noble metal
catalysts (Table S9, Supporting Information). The aliovalent
substitution of copper in ceria lattice is mainly responsible
for the abundant oxygen vacancies and the rapid reoxidation
of Ce3+ to Ce4+, resulting in the rapid oxidation of toluene to
CO2. 2D morphology of CeCuOx catalyst, exposing more (100)
crystallographic planes (indicated by the intense (200) diffrac-
tion peak in Figure 3c), also contributes to the higher reactivity
due to the lower formation energy of oxygen vacancy (i.e.,
2.27 eV of CeO2 (100) planes compared to 2.60 eV of CeO2
(111) planes).[40]
temperatures below 125 °C, while toluene and o-xylene require
higher oxidation temperatures (i.e., 200 °C).
The catalyst’s tolerance to moisture is important for many
applications. The CeCuOx catalyst was subjected to 69 200 ppm
H2O during a 60 h toluene oxidation reaction at 200 °C as
shown in Figure S13 (Supporting Information). The catalyst
suffered less than 1% decrease in conversion over the entire
period. It is speculated that the molecular-level mixing achieved
by the bimetallic MOF template synthesis is responsible for
the stability of the catalyst. CeCuOx exists in a single phase
and the copper was completely substituted into the ceria lattice
(Figure 3c). Furthermore, the exposed (100) crystallographic
planes of CeCuOx with 2D morphology might also contribute to
the higher stability compared to higher crystallographic planes
(e.g., (311)) found in CeO2 nanoparticles.
In summary, a novel strategy is introduced for preparing
active CeCuOx MTMO catalyst. The bimetallic MOF with mole-
cular mixing and spatial prearrangement of two kinds of metal
precursors promotes aliovalent substitution of copper (≈33%)
into ceria lattice, resulting in highly active and stable catalyst
+
3+


as attested from the obtained CeCuOx. The Cu OV Ce is
believed to be responsible for the catalyst’s high reactivity,[29]
enabling the CeCuOx to catalyze the oxidations of methanol,
acetone, toluene, and o-xylene at much lower temperatures.
This work also shows that it is possible to shape and form the
catalyst by manipulating the crystallization and growth of the
MOF catalyst precursors. The transformation from bimetallic
MOF to active MTMO catalyst, involving relatively mild thermal
treatment to remove the organic ligands, provides a simple
strategy to keep porous microstructures and low-dimensional
morphology from precursor to product. This approach pre-
serves the original 2D morphology that preferentially exposes
the (100) crystallographic planes where oxygen vacancies are
more readily created, enhancing the catalyst’s reactivity and
stability.
Figure 5b shows that CeCuOx catalyst is also highly active
for the oxidation of methanol, acetone, and o-xylene. The T50
is lowest for the catalytic combustion of methanol (80 °C),
followed by acetone (87 °C), o-xylene (142 °C), and toluene
(150 °C). It is generally accepted that the oxidation of organic
molecules over ceria catalysts follows a Mars–van Krevelen
mechanism.[10,41] The oxygenated aliphatic hydrocarbons react
faster than aromatic molecules. The latter requires the simulta-
neous abstraction of hydrogen from phenyl and methyl groups
on 4–5 adjacent oxygen sites and reaction to produce water.[41]
Thus, methanol and acetone were completely oxidized at
Besides providing a novel synthesis pathway for highly effi-
cient environmental catalysts applied to pollution treatment,
this approach can be readily used to prepare other bimetallic
catalysts or mixed oxide materials where molecular-level mixing
can enhance the material properties. This methodology can
afford the possibility of positioning the atoms in a prearranged
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Acknowledgements
The work was financially supported by the National Natural Science
Foundation of China/Research Grants Council Joint Research Scheme
[N_HKUST626/13], Research Grants Council—General Research
Fund [16307014], and Guangzhou Collaborative Innovation Key
Program [201704030074]. Parts of the experiments were conducted
on macromolecular crystallography (MX) and powdered diffraction
beamline at Australian Synchrotron (ANSTO).
Conflict of Interest
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The authors declare no conflict of interest.
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Keywords
2D materials, bimetallic MOF, CeCuOx microsheet, low-temperature
VOC remediation, transition metal oxide catalysts
Received: July 5, 2019
Revised: August 13, 2019
Published online:
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