Amorphous/Crystalline Hetero-Phase Pd Nanosheets: One-Pot Synthesis and Highly Selective Hydrogenation Reaction

Article abstract of DOI:10.1002/adma.201803234

Similar to heterostructures composed of different materials, possessing unique properties due to the synergistic effect between different components, the crystal-phase heterostructures, one variety of hetero-phase structures, composed of different crystal phases in monometallic nanomaterials are herein developed, in order to explore crystal-phase-based applications. As novel hetero-phase structures, amorphous/crystalline heterostructures are highly desired, since they often exhibit unique properties, and hold promise in various applications, but these structures have rarely been studied in noble metals. Herein, via a one-pot wet-chemical method, a series of amorphous/crystalline hetero-phase Pd nanosheets is synthesized with different crystallinities for the catalytic 4-nitrostyrene hydrogenation. The chemoselectivity and activity can be fine-tuned by controlling the crystallinity of the as-synthesized Pd nanosheets. This work might pave the way to preparing various hetero-phase nanostructures for promising applications.

Full text of DOI:10.1002/adma.201803234

CommuniCation
Hydrogenation
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Amorphous/Crystalline Hetero-Phase Pd Nanosheets:
One-Pot Synthesis and Highly Selective
Hydrogenation Reaction
Nailiang Yang, Hongfei Cheng, Xiaozhi Liu, Qinbai Yun, Ye Chen, Bing Li, Bo Chen,
Zhicheng Zhang, Xiaoping Chen, Qipeng Lu, Jingtao Huang, Ying Huang, Yun Zong,
Yanhui Yang, Lin Gu, and Hua Zhang*
different materials, which could com-
Similar to heterostructures composed of different materials, possessing
unique properties due to the synergistic effect between different components,
the crystal-phase heterostructures, one variety of hetero-phase structures,
composed of different crystal phases in monometallic nanomaterials are
herein developed, in order to explore crystal-phase-based applications. As
novel hetero-phase structures, amorphous/crystalline heterostructures are
highly desired, since they often exhibit unique properties, and hold promise
in various applications, but these structures have rarely been studied in noble
metals. Herein, via a one-pot wet-chemical method, a series of amorphous/
crystalline hetero-phase Pd nanosheets is synthesized with different crystal-
linities for the catalytic 4-nitrostyrene hydrogenation. The chemoselectivity
and activity can be fine-tuned by controlling the crystallinity of the as-syn-
thesized Pd nanosheets. This work might pave the way to preparing various
hetero-phase nanostructures for promising applications.
bine the advantages of individual compo-
[
2]
nents and/or overcome their drawbacks.
Similar to the heterostructures composed
of different materials, crystal-phase hetero-
structure, as one kind of hetero-phase
structures in monometallic nanomate-
rials, was proposed for the investigation
[
1c]
of crystal-phase-based applications
and also used for the epitaxial growth
of novel nanostructures for promising
[
3]
applications.
Different from crystals, amorphous
materials might exhibit intriguing prop-
[4]
erties and applications.
Inspired by
the crystal-phase heterostructures, it
is desired to develop a facile synthetic
strategy for preparation of the amor-
phous/crystalline hetero-phase noble
Noble metal nanocrystals have shown various promising appli-
cations in optics, catalysis, electronics, and so on, due to their
unique physiochemical properties, which strongly depend
on their shape, size, exposed facet, dimension, and atomic
arrangement.[ In order to prepare novel nanostructures
with multi-functionalities and/or enhanced properties, one of
the effective strategies is to synthesize heterostructures with
metal nanomaterials, which might exhibit unique properties
and promising applications compared to their amorphous and
crystalline counterparts. To the best of our knowledge, there
is rarely report on the preparation of amorphous/crystalline
hetero-phase noble metal nanomaterials, since it is quite chal-
lenging to obtain amorphous noble metal nanomaterials due
1]
[
5]
to the strong metallic interactions between the atoms.
Dr. N. Yang, H. Cheng, Q. Yun, Y. Chen, Dr. B. Chen, Dr. Z. Zhang,
Dr. Q. Lu, J. Huang, Dr. Y. Huang, Prof. H. Zhang
Center for Programmable Materials
Dr. B. Li, Dr. Y. Zong
Institute of Materials Research and Engineering
A*STAR (Agency for Science, Technology and Research)
School of Materials Science and Engineering
Nanyang Technological University
2 Fusionopolis Way, Innovis #08-03, Singapore 138634, Singapore
X. Chen
5
0 Nanyang Avenue, Singapore 639798, Singapore
School of Chemical and Biomedical Engineering
Nanyang Technological University
Singapore 637459, Singapore
E-mail: hzhang@ntu.edu.sg
X. Liu, Prof. L. Gu
Beijing National Laboratory for Condensed Matter Physics
Institute of Physics
Chinese Academy of Sciences
Beijing 100190, China
Dr. Y. Yang
Institute of Advanced Synthesis
School of Chemistry and Molecular Engineering
Jiangsu National Synergetic Innovation Center for Advanced Materials
Nanjing Tech University
X. Liu
School of Physical Sciences
University of Chinese Academy of Sciences
Beijing 100049, China
Nanjing 211816, China
Prof. L. Gu
Collaborative Innovation Center of Quantum Matter
Beijing 100190, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.201803234.
DOI: 10.1002/adma.201803234
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Figure 1. Characterization of the hetero-phase Pd nanosheets in Sample 1. a) TEM and b) DF-STEM images showing the nanosheet morphology.
c) C -corrected STEM-HAADF image showing the hetero-phase structure, in which the crystalline domains are marked by the dashed yellow curves.
S
d,e) The corresponding FFT patterns of the selected regions marked by the red and green squares, respectively, in (c). f) SAED pattern of Sample 1.
g) TEM image of a folded Pd nanosheet, showing the folded edge with measured width of ≈1.0 nm.
Herein, by using a one-pot wet-chemical method, the amor-
phous/crystalline hetero-phase Pd nanosheets have been
synthesized, which are then used as the catalysts for selective
hydrogenation of 4-nitrostyrene. The crystallinity of hetero-
phase Pd nanosheets can be controlled by fine-tuning the
experimental conditions. Importantly, with proper control over
the crystallinity of hetero-phase Pd nanosheets, their high cata-
lytic activity and excellent selectivity have been achieved.
by the bright spots (Figure 1d) and diffuse ring (Figure 1e) in
the corresponding selected-area fast Fourier transform (FFT)
patterns, respectively. For clarity, the crystalline domains are
marked by the dashed yellow curves (Figure 1c). Moreover,
the weak ring in the selected area electron diffraction (SAED)
pattern (Figure 1f) further reveals their poor crystallinity. The
average width of the folded edge in nanosheets was measured
to be ≈1.0 nm (Figure 1g), which corresponds to the thickness
[
7]
In a typical experiment, in order to prepare hetero-phase
Pd nanosheets with different crystallinities, the Pd(acac)2
was reduced by CO, which was released by the decomposi-
tion of Mo(CO)6,[ in octanoic acid at various temperatures
of nanosheets, proving their ultrathin structure.
Our synthetic method is quite general. By simply tuning
the synthetic temperature,
a series of hetero-phase Pd
6]
nanosheets with different crystallinities, referred to as Sam-
ples 2–5, are obtained at 52, 60, 80, and 100 °C, respectively.
All of the nanosheets (Figure 2a–d, Figure S1a–d, Supporting
Information) possess a similar thickness of ≈1.0 nm, meas-
ured from the folded edges in high-resolution TEM images
(Figure S1e–h, Supporting Information). It is ≈2.0 nm thinner
than the thickness measured by atomic force microscopy
(≈3.0 nm, Figure S2, Supporting Information), due to the octa-
noic acid molecules coating on the double-side surfaces of Pd
(48–100 °C) for 40 min (see the Supporting Information for
details). The Pd nanosheets obtained at 48 °C are denoted
as Sample 1 (Figure 1). Transmission electron microscopy
(
(
TEM) (Figure 1a) and dark-field scanning TEM (DF-STEM)
Figure 1b) images confirm that the nanosheets with high purity
were obtained. The spherical aberration-corrected STEM-high-
angle annular dark-field (C -corrected STEM-HAADF) image
S
(Figure 1c) shows that a typical nanosheet is composed of crys-
[
8]
talline and amorphous domains, which are further identified
nanosheets. The C -corrected STEM-HAADF images show
S
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Figure 2. Characterization of the hetero-phase Pd nanosheets with different crystallinities. a–d) DF-STEM and e–h) C -corrected STEM-HAADF images
S
of the hetero-phase Pd nanosheets in Samples 2–5, respectively. The crystalline domains are marked by the dashed yellow curves in (e), while the
amorphous domains are marked by the dashed light-blue curves in (f–h). i) XRD spectra of Samples 1–5. j) XPS spectra of Pd 3d for Samples 1–5.
k) The magnification of the normalized Pd(0) 3d3/2 peaks in Samples 1 to 5. l) The magnification of the normalized Pd(0) 3d5/2 peaks in Samples 1–5.
that these nanosheets are also composed of amorphous and
crystalline domains (Figure 2e–h), and the area of amorphous
domains decreases from Sample 2 to Sample 5. By measuring
the areas of the amorphous and crystalline domains in the
Pd(0) 3d5/2 peaks gradually shift to lower binding energy from
Sample 1 to Sample 5. This observation, i.e., the amorphous
nanosheet has a greater binding energy, is consistent with the
[
9]
previous report. Meanwhile, there is no signal from Pd(II),
indicating the metallic state of the synthesized Pd nanosheets.
Noble metals, such as Pd, have been widely used as the heter-
C -corrected STEM-HAADF images, the percentages of the
S
crystalline areas in Samples 1–5 are estimated to be ≈11%,
[10]
1
9%, 65%, 83%, and 96%, respectively. Moreover, the X-ray dif-
ogeneous catalysts for hydrogenation, which is very important
[10b,11]
fraction (XRD) was used to characterize the crystallinity of the
hetero-phase Pd nanosheets. As shown in Figure 2i, there is
no obvious diffraction peak for Sample 1, indicating that the
dominant phase of Sample 1 is amorphous, consistent with the
aforementioned results (Figure 1). The intensity of diffraction
peaks attributed to the face-centered cubic Pd (PDF# 46–1043)
increases from Sample 1 to Sample 5 (Figure 2i), indicating
the increased crystallinity, which has also been proved from
the increasingly intensive diffraction rings in the SAED pat-
terns in Samples 1–5 (Figure 1f, Figure S1i–l, Supporting
Information). Furthermore, X-ray photoelectron spectroscopy
in the industrial and fine chemical fields.
However, it is
very challenging to achieve the highly selective hydrogenation of
molecules containing both CC double bonds and nitro groups,
[10b,11]
since both functional groups can be readily hydrogenated.
Herein, as a proof-of-concept application, in order to investigate
the correlation between the chemoselectivity/catalytic activity
and the crystallinity of noble metal catalysts, the hetero-phase
Pd nanosheets are used in the hydrogenation of 4-nitrostyrene
[10d,12]
(NS), which is commonly used as the model reaction.
Figure 3a presents three products after the hydrogenation of
NS. The selective hydrogenation of CC bond generates the
4-nitroethylbenzene (EN), whereas the selective reduction of
nitro group produces the 4-aminostyrene (AS). Finally, the fully
(XPS) taken from Samples 1–5 was used to confirm their dif-
ferent electronic structures (Figure 2j–l). The Pd(0) 3d3/2 and
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Figure 3. Catalytic performance of the hetero-phase Pd nanosheets with different crystallinities. a) Hydrogenation reaction of NS. b,c) Time-dependent catal-
ysis in the hydrogenation of NS by using Samples 1 and 5, respectively, as the catalyst. Reaction conditions: room temperature, H balloon. d) Recycling tests
2
of Samples 2 and 4 in the hydrogenation of NS. Reaction conditions: room temperature, H balloon, 150 min (2.5 h) for Sample 2, 60 min (1 h) for Sample 4.
2
hydrogenated product is 4-ethylbenzenamine (EA). Gas chro-
matography (GC) was adopted to track the reaction process,
and the products were determined by GC-mass spectrometry
Pd nanosheets with the highest crystallinity among Samples
1–5, EN was the main product at the initial stage. With the
prolonged hydrogenation reaction, EN was gradually converted
to the fully hydrogenated product EA. After hydrogenation for
240 min (4 h), EA was the only product with a high purity over
99% (Figure S7, Supporting Information).
(GC-MS) and nuclear magnetic resonance spectroscopy. The
hydrogenation reaction was conducted at room temperature
under H2 atmosphere (see the Supporting Information for
details). The time-dependent catalytic curves for the hydrogena-
tion of NS by Samples 1 and 5 are shown in Figure 3b,c, respec-
tively, in which, the conversion curve (blue) describes the per-
centage of NS converted to the products, whereas the selectivity
curve (red, black or green) describes the molar ratio of the cor-
responding product (EN, EA, or AS) in the total products.
Based on our experimental results, it was found that
Sample 1 possessed the high selectivity (>99%) toward the gen-
eration of EN throughout the reaction (Figure 3b), and a 100%
conversion of NS was achieved after 240 min (4 h) of hydro-
genation (Figures S3–S5, Supporting Information). The excel-
lent selectivity toward EN (>99%) did not decrease even after
hydrogenation for 30 h (Figure S6, Supporting Information),
meaning that EN cannot be converted to the fully hydrogenated
product (EA) by using Sample 1 as the catalyst. In comparison,
in the hydrogenation reaction catalyzed by Sample 5, i.e., the
Besides the chemoselectivity, the catalytic activity is also
dependent on the crystallinity of the hetero-phase Pd nanosheets.
Compared with Sample 1, Sample 2 also exhibits high chemo-
selectivity (>99%) of EN, but its catalytic activity is higher than
Sample 1. It only takes 150 min (2.5 h) to achieve the 100% con-
version of NS (Figure S8, Supporting Information). To the best
of our knowledge, Sample 2 is the best heterogeneous catalyst to
hydrogenate the CC double bond under mild conditions with
high catalytic selectivity (>99%) and 100% conversion (Table S1,
Supporting Information). Comparing the time-dependent cata-
lytic curves of Samples 3–5 (Figure S9 and S10, Supporting
Information and Figure 3c), all of the catalysts produce the fully
hydrogenated product, i.e., EA. Among them, Sample 4 shows
the highest catalytic activity, which completely converted NS to
the fully hydrogenated product (EA) with a purity of >99% in
60 min (1 h, Figure S10, Supporting Information). Moreover,
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Samples 2 and 4 also exhibited good stability. The catalytic selec-
tivity did not decrease after recycling for five times (Figure 3d).
The time-dependent catalytic curves of Samples 1–5
AcRF Tier 1 (2016-T1-001-147; 2016-T1-002-051; 2017-T1-001-150; 2017-
T1-002-119), and NTU under Start-Up Grant (M4081296.070.500000). The
authors would like to acknowledge the Facility for Analysis, Characterization,
Testing, and Simulation, Nanyang Technological University, Singapore, for
their electron microscopy (and/or X-ray) facilities.
(Figure 3b,c, Figures S8–S10, Supporting Information) show
that the amorphous-dominant samples (Samples 1 and 2)
exhibit higher chemoselectivity toward the hydrogenation of
CC double bonds and lower catalytic activity, compared with
the crystalline-dominant samples (Samples 3–5). The possible
reasons can be explained as follows. 1) Sample 1 presents a
higher binding energy than Sample 5 (Figure 2j–l), indicating
the electron density around Pd atoms in amorphous phase
is lower than that in crystalline phase. It suggests the weaker
PdPd metallic bonds in amorphous phase, leading to a lower
surface energy.[ Therefore, the amorphous Pd surface would
preferentially adsorb and hydrogenate the groups with lower
polarity, i.e., CC bonds, but impede the hydrogenation of
the functional groups with higher polarity, i.e., nitro groups,
resulting in the excellent chemoselectivity in the amorphous-
dominant samples (Samples 1 and 2). 2) The lower electron
density around Pd atoms in the amorphous-dominant samples
Conflict of Interest
The authors declare no conflict of interest.
Keywords
amorphous, catalysts, chemoselective, hetero-phase, Pd nanosheets
13]
Received: May 21, 2018
Revised: June 29, 2018
Published online:
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In summary, we report a one-pot wet-chemical method
for the synthesis of amorphous/crystalline hetero-phase Pd
nanosheets. The crystallinity of these nanosheets can be easily
tuned by simply changing the reaction temperature. These
nanosheets are used as the heterogeneous catalysts to study the
chemoselectivity in hydrogenation of 4-nitrostyrene. It is found
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Acknowledgements
N.Y. and H.C. contributed equally to this work. This work was supported
by MOE under AcRF Tier 2 (ARC 19/15, Nos. MOE2014-T2-2-093;
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