Mesoporous Silica-Encaged Ultrafine Bimetallic Nanocatalysts for CO2 Hydrogenation to Formates

Article abstract of DOI:10.1002/cctc.201901167

CO₂ hydrogenation to formic acid/formate has been recognized as a key reaction to realizing the CO₂-mediated hydrogen energy cycle. Herein, ultrafine and well-dispersed Pd?CoO nanoparticles (～1.8 nm) were encapsulated within mesoporous silica nanospheres (MSNs) via a facile one-pot ligand-protected synthesis strategy. The MSN-encaged bimetallic nanocatalysts exhibit excellent catalytic activity and stability for the formate production from CO₂ hydrogenation, showing high turnover frequency value up to 1824 h^?1 at 373 K, which is among the top-level reported for heterogeneous catalysts.

Full text of DOI:10.1002/cctc.201901167

Accepted Article
Title: Mesoporous Silica-Encaged Ultrafine Bimetallic Nanocatalysts for
CO2 Hydrogenation to Formates
Authors: Qiming Sun, Xinpu Fu, Rui Si, Chi-Hwa Wang, and Ning Yan
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ChemCatChem
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COMMUNICATION
Mesoporous Silica-Encaged Ultrafine Bimetallic Nanocatalysts for
CO Hydrogenation to Formates
2
Qiming Sun,* [a, b] Xinpu Fu, Rui Si, Chi-Hwa Wang, and Ning Yan*
[a]
[c]
[b]
[b]
Abstract: CO
2
hydrogenation to formic acid/formate has been
-mediated hydrogen
Mesoporous silica nanospheres (MSNs) is regarded as
promising support for the confined synthesis of small MNPs due
to their well-defined channels, large surface area and pore volume,
and high thermal and chemical stability.32-40 Conventionally, the
metal species were introduced to the MSNs via an incipient
wetness impregnation method, but it often suffers from the
formation of large MNPs with non-uniform dispersion.[41, 42] Post-
modifying the MSNs with functional groups is an alternative
approach to stabilize the small-sized MNPs, but it is usually
subjected to intricate steps and increased the cost.[43] Thus,
developing more efficient synthesis methods, particularly the one-
pot protocol for the preparation of high-performance MSN-
encaged metallic catalysts is desirable. Recently, fabrication of
hybrid multi-MNPs, especially introducing cheap transition metal
species, has been proven effective to improve the catalytic activity
and simultaneously decrease the synthesis cost.[16, 44-46] To our
knowledge, until now, there has been no report of in situ
embedding ultrafine multi-MNPs within MSNs to promote the
recognized as a key reaction to realizing the CO
2
energy cycle. Herein, ultrafine and well-dispersed Pd-CoO
nanoparticles (~1.8 nm) were encapsulated within mesoporous silica
nanospheres (MSNs) via a facile one-pot ligand-protected synthesis
strategy. The MSN-encaged bimetallic nanocatalysts exhibit excellent
catalytic activity and stability for the formate production from CO
2
1
−
hydrogenation, showing high turnover frequency value up to 1824 h
at 373 K, which is among the top-level reported for heterogeneous
catalysts.
Stabilizing and mitigating the CO
atmosphere has become a major scientific, technological and
societal challenge of the 21 century.
attractive C1 resource for useful chemicals and fuels that alleviate
the dependence of carbon source from fossil fuels.[3-8] Recently,
2
concentration in the
st
[1, 2]
2
CO also serves as an
2
CO hydrogenation to formic acid/formate has been recognized
as a promising strategy, since the target product is a useful
chemical, and more importantly, is a potentially important platform
for chemical energy storage.[9-12]
The reaction can be achieved in both homogeneous and
heterogeneous catalytic systems. Since the first employment of a
2
catalytic performance of CO hydrogenation into formate. Here,
we developed a facile one-pot synthesis strategy to encapsulate
ultrafine and well-dispersed bimetallic Pd-CoO NPs inside the
MSN channels. Thanks to the ultrafine MNP sizes and electron-
enriched Pd species, the optimized Pd0.8Co0.2@MSN catalyst
exhibits excellent catalytic activity and stability for the formate
Ru complex stabilized by phosphine ligands for CO
2
hydrogenation into formate, homogeneous metal complexes have
been intensively investigated.[13, 14] More recently, heterogeneous
catalysts, especially supported metallic catalysts, joined the
arena.[15-31] Among variously reported metal catalysts, supported
Pd-based catalysts exhibited the best activity. Several types of
supports for metal nanoparticles (MNPs), such as active
2
production from CO , affording a turnover frequency (TOF) value
of 1824 h-1 at 373 K.
carbon,[
21, 23]
graphene,
[24, 25]
g-C
hydrogenation into formate reactions.
3 4
N
,[26, 27] and metal oxide,
[16, 28, 29]
have been employed in CO
2
However, these catalysts usually suffer from poor long-term
stability and/or poor recyclability due to the severe aggregation of
MNPs under high pressures and temperatures. It is desired but
challenging to develop catalyst with enhanced stability and
activity at the same time. The key is to identify protocols that
enable the formation of ultrafine MNPs that are resistant to
agglomeration.
x
Scheme 1. Schematic illustration of the preparation of Pd Co1-x@MSN catalysts.
As shown in Scheme 1, the MSN-encaged ultrafine bimetallic
nanocatalysts were synthesized by a one-pot ligand-protected
synthesis method. First, cetyltrimethyl ammonium bromide
[a]
[b]
[c]
Dr. Q. Sun
(
H
CTAB) was dissolved in water, and then a suitable amount of
NUS Environmental Research Institute (NERI), National University
of Singapore, 138602, Singapore
Dr. Q. Sun, Dr. X. Fu, Prof. C. Wang, Prof. N. Yan
Department of Chemical and Biomolecular Engineering, National
University of Singapore, 4 Engineering Drive 4, 117585, Singapore
Prof. R. Si
Shanghai Synchrotron Radiation Facility, Shanghai Institute of
Applied Physics, Chinese, Academy of Sciences, Shanghai 201204,
People’s Republic of China
-3
2
PdCl
4
/CoCl
2
solution (n(Pd+Co)/n(Si)=2.5×10 ) with different molar
ratios was added. Following that, aqueous ammonia was
introduced and the color of the solution immediately switches from
yellow to colorless, due to the formation of Pd(NH
Co(NH
stirring and heating at 60 ºC, the white solid product was collected
and underwent calcination and reduction treatments to offer the
2+
3
)
4
and
3+
3
)
6
complexes. Upon adding tetraethoxysilane (TEOS),
E-mail: erisq@nus.edu.sg; ning.yan@nus.edu.sg
Pd
atomic emission spectroscopy (ICP-AES) analyses suggest that
the Pd Co1-x@MSN samples possess a total metal (Pd+Co)
x
Co1-x@MSN catalyst samples. Inductively coupled plasma
Supporting information for this article is given via a link at the end of
the document.
x
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loading amount of 0.39-0.21 wt%, while the Pd/Co molar ratios of
The X-ray diffraction patterns of the samples are shown in
Figure 2A. The peak at ~1.8 was clearly observed, proving the
o
Pd
x
Co1-x@MSN (x= 0.8, 0.6, 0.4, and 0.2) samples are 0.78/0.22,
0
.61/0.39, 0.39/0.61, and 0.18/0.82, respectively, in accordance
existence of ordered mesoporous structures in the samples.
Moreover, no peak belonging to metal species was detected in
metal-containing samples due to the lower metal loading and the
small diameter of MNPs (Figure S2 in the Supporting Information).
with predesigned ratios (Table S1). For comparison purposes, Pd
catalyst synthesized by impregnation method (denoted as
Pd/MSN-im) and Pd MNPs supported on solid silica spheres
(
denoted as Pd/SiO
Transmission electron microscope (TEM) images of Pd
@MSN samples are shown in Figures 1A-F and S1 in the
2
) were also prepared. Main Text Paragraph.
N
2
adsorption measurements show that the additionally
x
Co1-
introduced metal species occupy some spaces of MSNs, leading
to moderate decrease (~10%) of the BET surface area and
mesoporous volume (Figure 2B and Table S2 in the Supporting
Information). Nevertheless, the Pd@MSN and Pd0.8Co0.2@MSN
still possess sufficient and large void space (~601 m /g and ~0.72
cm /g) for the reactant to diffuse and transfer. Pore-size
x
Supporting Information. The MSNs with a diameter of about 50
nm and ordered mesoporous channels (~3 nm) were clearly
observed. Ultrafine MNPs were encapsulated inside the
mesopores of MSNs and uniformly distributed throughout the
MSNs. The average sizes of MNPs within MSNs are 1.7-1.9 nm,
which are much smaller than those of Pd/MSN-im (using the
2
3
distributions of samples further demonstrate the existence of
ordered mesopores (~ 2.9 nm) in accordance with the TEM
wetness impregnation method, 4.4 nm) and Pd/SiO
2
(Pd NPs
observation.
H
2
-temperature-programmed
reduction
loaded on the solid silica spheres synthesized by the wetness
impregnation method, 13.8 nm). Notably, all MNPs are located
within the MSN matrix, and can be hardly observed on the outer
surface of MSNs. It indicates that the one-pot synthetic strategy
effectively encages the metal species inside the ordered
mesoporous channels of MSNs, which can be expected to
improve the ability of MNPs by inhibiting aggregation under harsh
conditions. The elemental mapping images of respective sample
Pd0.8Co0.2@MSN and corresponding energy-dispersive X-ray
measurements reveal that the Pd atoms was reduced below 50 º
C, and a negative peak at 75 ºC was also observed for all Pd-
containing samples attributed to a hydrogen evolution from β-
hydride decomposition (Figure S3 in the Supporting
Information).42 Notably, the introduction of Pd species significantly
decreases the reduction temperature of cobalt oxides, which is
indicative of the formation of bimetallic NPs. X-ray photoelectron
spectroscopy (XPS) spectra of samples are shown in Figure S4
in the Supporting Information. The peaks at 335.2 and 340.5 eV
in the Pd 3d region and 780.6 eV in the Co 2p region presented
in the spectra of Pd0.8Co0.2@MSN correspond to the 3d5/2 and
(
EDX) spectrum reveal uniform distributions of Si, O, Pd, and Co
throughout MSNs (Figure 1G).
3d
3/2 of metallic Pd0 and the 3P3/2 of CoO, respectively.[43]
Compared to the Pd@MSN, the Pd 3d peaks of Pd0.8Co0.2@MSN
are shifted to lower binding energies, indicating the Pd atoms of
Pd0.8Co0.2@MSN are more electron-enriched.
2
Figure 2. A) XRD patterns and B) N adsorption isotherms of pure MSN,
Pd@MSN, and Pd0.8Co0.2@MSN samples. C) Pd K-edge XANES profiles of Pd
foil, PdO, Pd@MSN, and Pd0.8Co0.2@MSN samples. D) FTIR spectra of CO
adsorbed on samples.
Figure 1. TEM images of A, B) Pd@MSN, D, E) Pd0.8Co0.2@MSN, and C, F)
To further identify the structure of the metal species, X-ray
absorption near-edge structure (XANES) and extended X-ray
absorption fine structure (EXAFS) spectroscopies were carried
out. The Pd K-edge XANES profiles of Pd@MSN and
Pd0.8Co0.2@MSN strongly resembled that of the Pd foil, meaning
corresponding particle size distribution. Surface-weighted mean cluster
i i i i i
d d is the number of crystallites possessing
3/Σn 2, where n
diameter dTEM = Σn
diameter d. (G) TEM image of Pd0.8Co0.2@MSN, EDX spectrum, and the
corresponding EDX mapping images for various elements.
i
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the valence of Pd in these two samples is zero (Figure 2C).
Notably, compared with Pd@MSN, a small edge shift to lower
energy for Pd0.8Co0.2@MSN was observed. It demonstrates that
the Pd atoms in Pd0.8Co0.2@MSN possess a higher electron
density than that in Pd@MSN, in full agreement with the result of
XPS measurement. In the Pd K-edge Fourier-transformed EXAFS
spectra, the peak at ~1.92 Å attributed to Pd-O bond was
observable in Pd@MSN, but hardly detected in Pd0.8Co0.2@MSN,
further substantiate that the Pd in the Pd0.8Co0.2@MSN is electron
enriched (Figures S5 and S6 in the Supporting Information). The
average coordination numbers of the Pd-M (M = Pd or Co) bonds
in Pd@MSN and Pd0.8Co0.2@MSN are 3.8±0.9 and 5.0±1.0,
respectively. Considering the similar particle sizes of MNPs in
Pd@MSN and Pd0.8Co0.2@MSN, both Pd-Pd and Pd-Co bonds
are most likely formed in the Pd0.8Co0.2@MSN. Note that the sizes
of MNPs obtained based on EXAFS data, if considering the
contributions from all the species of Pd-O, Pd-Cl and Pd-M (M =
Pd or Co), are 1.5 and 1.6 nm for Pd@MSN and Pd0.8Co0.2@MSN,
respectively (Table S3 footnotes in the Supporting Information),
matching those obtained from the TEM images. In situ CO-DRIFT
measurements were further employed to characterize the
electronic state of metal species. As shown in Figure 2D, the peak
up to 4082 after 10 h (Figure S11 in the Supporting Information).
Note that the Co@MSN and pure MSN catalysts show no catalytic
activity. The improvement of catalytic performance over
Pd0.8Co0.2@MSN is mainly attributed to the electron-enriched Pd
surfaces due to the electron transfer from CoO to Pd, which
facilitates the hydrogenation of bicarbonate, in accordance with
previous work.[
16]
-1
at 2090 cm attributed to the linear-adsorbed CO on Pd sites was
observed in Pd@MSN, but this peak shifted to lower
a
-1
wavenumber in Pd0.8Co0.2@MSN (2084 cm ). Generally, the CO
frequency shift could be mainly due to the formation of some
larger metal particles or/and more reduced particles. Considering
the similar sizes of Pd and PdCo nanoparticles based on TEM
observations, the CO frequency shift can be mainly attributed to a
result of electronically more enriched Pd species in
Pd0.8Co0.2@MSN.[16] Moreover, the peak at 1920 cm attributed to
the bridging-adsorbed CO on Pd sites in Pd0.8Co0.2@MSN is much
stronger than that of Pd@MSN. The increased bridge-type
-1
Figure 3. A) Catalytic activity and B) TOF value of various catalysts. Reaction
adsorbed CO is ascribed to the electronic effect of Pd-CoO NPs:
the electron-enriched Pd atoms have a stronger CO adsorption.[49]
conditions: 2 mL of H
at 373 K. C) NaHCO
different temperatures. D) Recycling tests of the Pd0.8Co0.2@MSN for NaHCO
hydrogenation. E and F) CO hydrogenation reactions under different initial
pressures (H /CO = 1:1) and KHCO concentration at 373 K catalyzed by the
Pd0.8Co0.2@MSN catalyst.
2
3
O containing 2 mmol of NaHCO , 10 mg catalyst, 2.0 MPa
H
2
3
hydrogenation of catalyzed by the Pd0.8Co0.2@MSN at
Catalytic activities for the hydrogenation of NaHCO
3
solution (1
3
M, 2.0 MPa H ) to HCOONa at 373 K catalyzed over various
2
2
catalysts are presented in Figure 3A. The HCOONa generation
rate over the Pd0.8Co0.2@MSN reaches up to 408 molFA molPd-1
-
2
2
3
h
1,
which are about 2-, 4- and 10-time higher than that over
respectively.
Significantly, Pd0.8Co0.2@MSN shows more than 2-fold higher
activity than those of commercial Pd/Carbon, Pd/BaSO , and
Pd/Al catalysts (the TEM images of these catalysts are shown
in Figures S7-S9 in the Supporting Information). According to the
-pulse adsorption results (Table S1 and S10 in the Supporting
Information), some H adsorption can be observed on the
Pd@MSN and Pd0.8Co0.2@MSN, but undetected in Co@MSN,
indicating the Co surface atoms do not participate in H
2
Pd@MSN, Pd/MSN-im, and Pd/Solid SiO ,
Kinetic
studies
of
NaHCO
3
hydrogenation
over
Pd0.8Co0.2@MSN are also investigated by varying concentrations
4
-
-
of the H
as a constant) at 373 K. The formate generation rate is dependent
on both concentrations of HCO -
and H (Figures S12 and S13 in
2 3 2 3
and HCO (keeping either the H or HCO concentration
2
O
3
3
2
H
2
the Supporting Information). The logarithmic plots of the
2
-
calculated reaction rate versus HCO
3
and H
2
concentration give
slopes of 1.53 and 0.42, respectively, indicating that the
concentration of HCO -
is more sensitive to the reaction rate than
. Previous work has shown that the adsorption of
by a dissociated H
2
3
chemisorption. The Pd dispersion of Pd0.8Co0.2@MSN is 52.5 %,
similar to that of Pd@MSN (48.8 %), suggesting that the
introduction small amount of CoO does not jeopardize the
exposure of active Pd sites. However, introducing more CoO
that of the H
2
-
and the attack on the C atom in HCO -
HCO
3
3
atom is the rate-determining steps in the hydrogenation reaction,
rather than the dissociation of H
.[16] Our kinetic investigations
2
induced
a
decrease of Pd dispersion. Compared with
Co1-x@MSN catalysts
agreed with the above conclusion. Not unexpected, the reaction
was accelerated with the increase of temperatures. The apparent
monometallic Pd@MSN, the bimetallic Pd
x
displayed significantly enhanced catalytic activities (Figure 3B).
Notably, the Pd0.8Co0.2@MSN exhibited the highest TOF value of
activation energy (E
mol , slightly lower than that of Pd@MSN (36.6 kJ mol ) (Figures
a
) of Pd0.8Co0.2@MSN is estimated as 33.2 kJ
-1
-1
-1
7
77 h , which represents a nearly 2-fold enhancement than that
3C and S14 in the Supporting Information).
of Pd@MSN. The turnover number of Pd0.8Co0.2@MSN reaches
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COMMUNICATION
Of note, the catalytic activity of Pd0.8Co0.2@MSN remained [2] J. Hansen, M. Sato, R. Ruedy, K. Lo, D. W. Lea and M. Medina-Elizade,
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unchanged (Figure S15 in the Supporting Information). In
comparison, the catalytic activity of Pd/MSN-im catalyst
decreased by about 40% after 5 cycles due to the increasing size
of Pd NPs (Figure S16 and S17 in the Supporting Information). It
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using different bicarbonates and carbonates as feedstock. The
hydrogenation efficiency of bicarbonates is much higher than that
of carbonates (Figure S18 in the Supporting Information). When
different bicarbonate was employed, the Pd0.8Co0.2@MSN
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exhibits a TOF of 1285 h using the KHCO
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1
[
improved along with the increase of concentrations of KHCO
initial pressures (as shown in Figures 3E and F). A TOF value of
824 h-1 was achieved under a total pressure of 2.0 MPa (H
/CO
1:1) and 2 mL of KHCO
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Acknowledgements
[
We thank the NUS Flagship Green Energy Programme and the
National Research Foundation, Prime Minister’s Office,
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Technological Enterprise (CREATE) programme, and the
National Science Foundation of China (NSFC) (grant nos.
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[
2
Keywords: heterogeneous catalysis • CO hydrogenation •
Dogu, Appl. Catal. B: Environ., 2009, 92, 250-261.
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ChemCatChem
10.1002/cctc.201901167
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Ultrafine and well-dispersed Pd-CoO
Qiming Sun,* Xinpu Fu, Rui Si, Chi-Hwa
Wang, and Ning Yan*
nanoparticles
(~1.8
nm)
were
encapsulated within mesoporous silica
nanospheres (MSNs) via a facile one-
Page No. – Page No.
pot
strategy. The MSN-encaged bimetallic
nanocatalysts exhibit enhanced
catalytic activity and stability for the
formate production from CO
ligand-protected
synthesis
Mesoporous Silica-Encaged Ultrafine
Bimetallic Nanocatalysts for CO
Hydrogenation to Formates
2
2
hydrogenation compared with control
samples, showing high turnover
−
1
frequency values up to 1824 h at 373
K.
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