Ultra-small Pd nanoparticles derived from a supramolecular assembly for enhanced electrochemical reduction of CO2 to CO

Article abstract of DOI:10.1039/c9cc02393c

Ultra-small Pd nanoparticles were successfully prepared through reduction of a supramolecular assembly formed between the macrocyclic decamethylcucurbit[5]uril (Me₁₀CB[5]) and [PdCl₄]^2- anions via H-bonds. The final Pd NPs exhibit excellent activity and stability toward the electrochemical reduction of CO₂ to CO. This work provides a new strategy for the preparation of nanocatalysts.

Full text of DOI:10.1039/c9cc02393c

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Ultra-small Pd Nanoparticles Derived from Supramolecular
Assembly for Enhanced Electrochemical Reduction of CO to CO
2
Received 00th January 20xx,
Accepted 00th January 20xx
ab
b
ab
b
b
b
Ruru Chen, Minna Cao,* Weiguang Yang, Huimin Wang, Suyuan Zhang, Hongfang Li* and
b
Rong Cao*
DOI: 10.1039/x0xx00000x
Ultral-small Pd nanoparticles were successfully prepared through that the intermediates of CO
2
RR are inclined to bind strongly
reduction a supramolecular assembly formed between the on the surface of smaller particles than that of larger particles
2
-
5
macrocyclic decamethylcucurbit[5]uril (Me10CB[5]) and [PdCl
4
]
due to d-band narrowing. However, small NPs tend to
anions via H-bonds. The final Pd NPs exhibit excellent activity and aggregate into bulk material because of the high surface
stability toward electrochemical reduction of CO
provides a new strategy for the preparation of nanocatalysts.
2
to CO. This work energy, resulting in the loss of activity during the continuous
6
electrochemical procedure. It’s an effective pathway to
weaken NPs aggregation through loading the metal NPs on
3
b
Electrochemical reduction of carbon dioxide (CO
2
RR) has substrates, but it’s difficult to obtain ultra-small and well-
attracted much more attention because the conversion of defined NPs using the normal metal precursors. On the other
carbon dioxide into value-added chemicals is a promising hand, the strong reductant (such as sodium borohydride and
alternative energy conversion and storage technology for the ethylene glycol) causes metal NPs growth rapid due to the fast
1
future. However, the thermodynamic stability and the low reduction rate, while the additional additive (such as sodium
7
electron affinity of CO
(
2
require large activation energy barrier citrate or surfactant PVP) tends to block the active sites.
2
i.e., overpotential) during the reduction process. Moreover, Cucurbit[n]urils (CB[n]) are pumpkin-shaped macrocyclic
the competitive reaction hydrogen evolution reaction (HER) molecules which are composed of glycoluril units and each
always outcompetes the CO RR as a result of the kinetic possess a hydrophobic cavity accessible through both identical
advantages, decreasing the selectivity of target product.
Therefore, exploring an electrocatalyst with good activity and (Me10CB[5]) is one of the smallest members of cucurbit[n]urils
2
3
8
carbonyl-fringed
portals.
Decamethylcucurbit[5]uril
selectivity is highly desirable in order to realize the CO
industrialization.
2
RR family, which have strong coordination ability with metal
9
atoms. Herein, a well-defined ultra-small Pd NPs (denoted as
Pd-based nanocatalysts have received much more attention Pd-Me10CB[5]) ca. 2.4 nm were obtained through the thermal
for producing CO-rich syngas due to the stronger binding reduction of the as-prepared supramolecular assembly
4
2-
energies for the reaction intermediates. However, the (composed of [PdCl
4
]
and Me10CB[5]) in the hydrogen
maximum FEco only reaches 60% for commercial Pd/C with an atmosphere. The unique confinement effect provide by the
4
a
2
overpotential larger than 490 mV in the CO RR. In addition, supramolecular assembly as well as the special steric effect
the support of commercial Pd/C is easily be etched, decreasing offered by macrocyclic Me10CB[5] suppressed the growth of Pd
the stability of catalyst. Therefore, it is challenging to improve NPs during the reduction process. In addition, the ordered
the CO selectivity and stability of Pd-based nanocatalysts crystalline structure of the supramolecular assembly assured
especially at low overpotential. Previous reports demonstrated
a.
b.
College of Chemistry, Fuzhou University, Fuzhou 350108, China.
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China.
Corresponding authors E-mail addresses: mncao@fjirsm.ac.cn (M. Cao),
*
hongfangli@fjirsm.ac.cn (H. Li), rcao@fjirsm.ac.cn (R. Cao).
Electronic Supplementary Information (ESI) available: TEM images, PXRD, size
distribution, XPS survey and Pd 3d high resolution spectra of the as-prepared
Pd/Me10CB[5] and Bulk Pd. Cyclic Voltammetries recorded in supporting
atmosphere at a scan rate of 100 mV s- with all
1
electrolyte (KOH 2 M) under N
2
the samples at room temperature. Crystal data and structure refinement of
supramolecular assembly can be seen in the Table S1. See DOI: 10.1039/x0xx00000x
Scheme 1 Schematic illustration of the fabrication of Pd-Me10CB[5].
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DOI: 10.1039/C9CC02393C
4
Fig. 1 The one-dimensional chain subunit of PdCl -Me10CB[5].
the final Pd NPs uniformly distributed on the intact Me10CB[5]
which both as stabilizer and support. The final Pd NPs exhibit
excellent catalytic activity and selectivity toward CO RR to CO.
2
Meanwhile, the final Pd NPs also show an enhanced stability of
a period longer than 12 h continuous test without obvious loss
of activity and selectivity. The results demonstrate using
supramolecular assembly as precursor to prepare ultra-small
NPs is a new strategy for the preparation of nanocatalysts for
CO
Initially, [(H
supramolecular assembly, denoted as PdCl
2
RR with high catalytic performance.
Fig. 2 (a) The TEM image of the Pd-Me10CB[5]; inset: size distribution of Pd NPs. (b)
The HRTEM image of the Pd-Me10CB[5]; inset: the FFT pattern. (c) The PXRD of Pd-
Me10CB[5]. (d) The Pd 3d high-resolution spectrum of Pd-Me10CB[5].
3
O) (Me10CB[5]@H O)]•[PdCl
2
2
4
]•3H
2
O, the unique
4
-Me10CB[5], was
obtained through diffusion method under acidic media in the
H-shape tube (Scheme 1). The crystalline structure of the
supramolecular assembly was analyzed by single crystal X-ray
diffraction (SI, Table S1 for structural data), which reveals that
the supramolecular assembly is composed of Me10CB[5] and
Me10CB[5] is thermally stable, as evidenced by the TGA result
that the structure of the macrocyclic structure maintained in
well below 350 °C (Fig. S4). The intact structure of Me10CB[5] is
also evidenced by the XRD pattern of the final obtained Pd-
Me10CB[5], acting both as stabilizer and support for the
reduced Pd NPs. More important, the ordered crystalline
structure of the supramolecular assembly provides a unique
special confinement effect for preventing the growth of the
reduced Pd NPs (Fig. S5). To further prove the advantages of
choosing such supramolecular assembly as precursor to
prepare ultra-small and uniformly distributed Pd NPs, control
experiment has been carried out. The TEM images
demonstrated that lack of hydrogen bonding and ordered
crystalline structure, reducing the physical mixture of
2
-
[
4
PdCl ] . The Me10CB[5] is bicapped by two hydronium ions
through H-bonds on both of its portals with a water molecule
encapsulated in its cavity (Fig. 1), which is typical and similar to
1
0
the most supramolecular structure composed of Me10CB[5].
2
-
The [PdCl
4
]
4
anions in PdCl -Me10CB[5] are located in the
middle of the adjacent Me10CB[5] molecules and formed a
one-dimensional (1D) supramolecular assembly by hydrogen
bonding between the oxygen from the bicapped portal water
2
-
molecules and the chlorine of the [PdCl
4
] (the average O ··· Cl
distance of 3.299 Å falls in the range of hydrogen bonds
1
1
between oxygen and chloride). The powder X-ray diffraction
PXRD) results verified the phase purity and the homogeneity
2
Me10CB[5] and PdCl only obtained Pd NPs with irregular
distribution and uneven size distribution (ca. 4.2 ± 0.8 nm) (Fig.
S7). In addition, only bulk Pd was obtained when reducing
(
of the obtained supramolecular assembly (Fig. S1).
The final Pd nanocatalyst was obtained through thermal
2
PdCl without the addition of Me10CB[5], indicating that the
reduction of the as-prepared supramolecular assembly in H
2
important protection from Me10CB[5] for Pd NPs. The results
demonstrate that the as-prepared supramolecular assembly is
the key to prepare well-defined Pd NPs in this work.
The X-ray photoelectron spectroscopy (XPS) was employed
to investigate the surface chemical component of the final Pd
catalysts. The representative XPS survey spectrum of the Pd-
Me10CB[5] clearly displays the existence of C, N, O, and Pd
elements (Fig. S9). The high resolution XPS spectrum of Pd 3d
orbital reveals that the coexistence of Pd (0) and Pd (II) (Fig.
atmosphere under 300 C for 3 h. As shown in Fig. 2a, the
transmission electron microscope (TEM) image illustrates the
final obtained near spherical ultra-small Pd NPs are uniformly
distributed on the substrate with an average size of 2.4 nm.
The energy dispersive spectrometer (EDS) mapping of the Pd-
Me10CB[5] was also measured, shown in the Fig S6. The
uniform distribution of C, N, O also indicate that the Pd NPs
loaded on the Me10CB[5]. The high-resolution TEM (HRTEM)
image shows that visualized lattice fringe spacing 0.226 nm
can be assigned to the Pd (111) plane (Fig. 2b). Fig. 2c shows
the XRD patterns of three samples, ie., the macrocyclic
molecules Me10CB[5], the as-prepared supramolecular
2
d). The peaks at binding energies of 335.4 eV and 340.7 eV
correspond to 3d5/2 and 3d3/2 of metallic Pd (0), respectively.
The two peaks with low intensity that located at 337.1 eV and
3
42.4 eV are attributed to the Pd (II). Comparing the area and
assembly PdCl
4
-Me10CB[5] and the final obtained Pd-
the peak intensity of the two pairs of peaks, it can be verified
that the metallic Pd is the predominant species.
Me10CB[5]. The broaden peak appears at 40.1 is ascribed to
1
2
the (111) plane of metallic fcc Pd (JCPDS no. 46-1043). The
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excellent stability. In this work, we test the stability of the
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DOI: 10.1039/C9CC02393C
2
catalyst using potentiostatic electrolysis at −0.6 V in CO -
3
saturated 0.5 M KHCO solution (Fig. 3d). The Pd-Me10CB[5]
catalyst exhibits remarkable stability without obvious
degradation of FE (red dots) and current density (black trace)
for CO during 12 h test. This could be account for the Pd NPs
anchored by Me10CB[5], preventing the accumulation of the Pd
NPs during the continuous reaction. After the long-term test,
TEM was employed to characterize the morphology of the Pd-
Me10CB[5] catalyst. It was found that Pd NPs kept its initial
morphology without obvious aggregation (Fig. S17 and 18).
This long-lasting stability with high FEco facilitates the practical
application of catalyst in energy and conversion devices like
2 2
zinc-CO batteries and CO splitting batteries. Previous studies
Fig. 3 Electrochemical CO
KHCO solution. (a) LSV measured in CO
.5 M KHCO solution, respectively. (b) FEs of CO, H
saturated 0.5 M KHCO solution. (c) Partial current density of CO, H
measured in CO -saturated 0.5 M KHCO solution. (d) Stability performance
operated at -0.6 V for 12 h in CO -saturated 0.5 M KHCO solution.
2
reduction over the Pd-Me10CB[5] electrode in 0.5 M
-saturated (red) and N -saturated (blue)
and HCOOH measured in CO
and HCOOH
demonstrated that the cucurbit[n]uril family shows highly
15
3
2
2
selective CO
sorption than CO. Therefore, we deduce that
2
0
3
2
2
-
the intact macrocyclic Me10CB[5] molecules not only offer
good protection for Pd NPs, but also benefit to adsorb
3
2
2
3
substrates (CO
immediately. Furthermore, the key role of Me10CB[5] offering
RR were performed moderate protection and direct effect on the sorption
in a standard three-electrode H-type cell with 0.5 M KHCO
procedure also guarantees the good catalytic performance of
aqueous solution as electrolyte. Fig. 3a shows the LSVs of Pd- Pd-Me10CB[5] for CO RR.
Me10CB[5]. In an N atmosphere, significant cathodic current
The good catalytic performance of Pd-Me10CB[5] was
2
) and release the aimed products (CO)
2
3
The activities of the catalyst toward CO
2
3
2
2
was recorded at the potential negative than –0.45 V, indicating further highlighted by control experiments. Pd/Me10CB[5],
a high overpotential for HER. The current density (normalized bulk Pd (TEM images shown in Fig. S7 and XPS shown in Fig.
by the geometrical surface area) measured in the CO
saturated solution starts to increase at -0.36 V owing to the the same experimental condition with Pd-Me10CB[5]. Fig. 4a
donated of CO
RR. The result is further evidenced by detecting shows the LSVs measured in the CO -saturated 0.5 M KHCO
2
-
S11-13) and commercial Pd/C (10 wt%) were measured under
2
2
3
the products after short-term potentiostatic electrolysis. solution. It is clear that the current density of Pd-Me10CB[5]
Usually, the Faradaic efficiency for CO (FEco) can be achieved presents sharper incremental tendency than the other samples.
8
5% when the potentials negative than -0.6 V for Pd-based Commercial Pd/C presents a moderate selectivity (the
4
a
catalysts. But the FEco of Pd-Me10CB[5] reaches 86% at -0.5 V, maximum FEco of 54.0% at -0.6 V) for CO in all the applied
4
a
which is an extremely high selectivity for CO at such lower potentials (Fig. 4b), which is in agreement with shao’s report.
overpotential.4
a, 13
In particular, the maximum FEco reaches In contrast, Pd/Me10CB[5] exhibits high FEco over 84% in the
9
2.5% at -0.6 V, outperforming most of the reported Pd-based wide potential range of -0.6 V to -0.9 V, which is almost twice
electrocatalysts at comparable overpotential (Table S5).
Besides, it is noted that all the FEco exceed 86% in a wide
potential range of -0.5 V to -0.9 V, indicating the superior
selectivity for CO formation of the catalyst. The results
demonstrated that Pd-Me10CB[5] shows an extremely high
selectivity at low overpotential. Furthermore, the CO current
2
density increases sharply than HCOOH and H with larger
overpotential applied to the catalyst, indicating the
accelerated reaction rate for the production of CO at higher
overpotential (Fig. 3c). Taking -1.0 V as reference potential, the
CO current density based on the geometric area reaches 17.91
-2
-2
mA cm and 0.329 mA cm based on the ECSA, respectively
Fig. S16). The current densities of H and HCOOH have slightly
(
2
changed due to the poor selectivity. The ECSA of Pd-Me10CB[5]
was also calculated based on reduction peak of the PdO (Fig.
S14b) formed in the positive scan of CV in N
2
-saturated 2 M
2 3
Fig. 4 (a) LSVs measured in CO -saturated 0.5 M KHCO solution of all the
samples. (b) The corresponding FEco of Pd-Me10CB[5] (black),
Pd/Me10CB[5] (red), Pd/C (green) and bulk Pd (blue), respectively. (c) The
maximum value of FEco for all the samples. (d) The CO current density of
all the samples.
KOH solution. As expected, Pd-Me10CB[5] displays high ECSA
2
-1
value of 51.42 m g , confirming the high density of low-
1
4
coordinated surface atoms (Fig. S15). Considering the
practical application, the catalyst was required superior
activity and selectivity to target products, but also have
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than that of commercial Pd/C. In particular, the maximum FEco
of Pd/Me10CB[5] reaches 91.8% at -0.7 V, verified that the key
role of Me10CB[5] as support. According to previous reports,
the cucurbit[n]urils have polar interaction with some small
Notes and references
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DOI: 10.1039/C9CC02393C
1
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5a, 16
molecules, such as CO
2
and CO.
We deduced that the
Me10CB[5] may participate in the electrochemical process
directly, which have been reported in a few cucurbit[n]uril
2
3
1
7
related electrocatalytic procedures. The selectivity of Pd-
Me10CB[5] would be further improved especially in the low
applied potential with the FEco reaches 86.6% at -0.5 V. We
hypothesized that the small sizes increased the binding energy
4
5
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for reaction intermediates except the influence of Me10CB[5],
indicating the importance of supramolecular assembly as
precursor for synthesis the electrocatalyst.
The CO current density of all the samples increases with the
overpotential except bulk Pd with poor selectivity for CO (Fig.
1
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d). Among them, the CO current density of Pd-Me10CB[5]
increases faster than that of the control samples, indicating
the accelerated formation of CO at higher overpotential.
Taking -1.0 V as reference potential, Pd-Me10CB[5] exhibits
appropriately 2- and 1.8-fold enhancement of CO current
density relative to Pd/Me10CB[5] and commercial Pd/C,
respectively. Besides, the ECSA value of Pd-Me10CB[5] (51.42
1
1378-11386.
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7
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2
-1
2 -1
m g ) is much higher than that of Pd/Me10CB[5] (30.09 m g )
2
-1
and bulk Pd (1.53 m g ), owing to small sizes confinement by
supramolecular assembly. The increased ECSA should be
8
9
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the rigid macrocyclic Me10CB[5] have been successfully
1
1 A. K. Ghosh, D. Ghoshal, J. Ribas, G. Mostafa and N. R.
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2
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1
1
-2
V and much higher partial current density (17.9 mA cm at -1.0
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4 Y. X. Chen, A. Lavacchi, S. P. Chen, F. Benedetto, M.
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-2
of 91.8% at -0.7 V and CO current density of 8.96 mA cm at -
.0 V) and most reported Pd-based catalysts. Control
1
1
experiments further highlight the crucial role of both
supramolecular assembly as precursor and Me10CB[5] support
in adjusting the adsorption for the substrates and release the
aimed products. These results open a new way to construct
nano-sized palladium catalysts for drastically enhanced the 16 T. C. Lee and O. A. Scherman, Chem. Commun., 2010, 46,
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2
2
performance of CO RR.
1
We appreciate the financial support from the National Key
R&D Program of China (2017YFA0206800, 2017YFA0700100),
the NSFC (21573238, 21571177, 21520102001), Key Research
Program of Frontier Science, CAS (QYZDJ-SSW-SLH045) and
5
“Strategic Priority Research Program” of the Chinese Academy
of Sciences (XDB20000000).
Conflicts of interest
There are no conflicts to declare.
4
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