Ordered Porous Pd Octahedra Covered with Monolayer Ru Atoms

Article abstract of DOI:10.1021/jacs.5b08956

Monolayer Ru atoms covered highly ordered porous Pd octahedra have been synthesized via the underpotential deposition and thermodynamic control. Shape evolution from concave nanocube to octahedron with six hollow cavities was observed. Using aberration-corrected high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy, we provide quantitative evidence to prove that only a monolayer of Ru atoms was deposited on the surface of porous Pd octahedra. The as-prepared monolayer Ru atoms covered Pd nanostructures exhibited excellent catalytic property in terms of semihydrogenation of alkynes.

Full text of DOI:10.1021/jacs.5b08956

Communication
pubs.acs.org/JACS
Ordered Porous Pd Octahedra Covered with Monolayer Ru Atoms
Jingjie Ge,† Dongsheng He,† Lei Bai,† Rui You,† Haiyuan Lu,† Yue Lin,† Chaoliang Tan,∥ Yan-Biao Kang,†
Bin Xiao,† Yuen Wu,† Zhaoxiang Deng,† Weixin Huang,† Hua Zhang,∥ Xun Hong,*,† and Yadong Li*,†,‡,§
†Center of Advanced Nanocatalysis (CAN), University of Science and Technology of China, Hefei, Anhui 230026, China
§
‡Department of Chemistry and Collaborative Innovation Center for Nanomaterial Science and Engineering, Tsinghua University,
Beijing 100084, China
∥Center for Programmable Materials, School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang
Avenue, Singapore 639798, Singapore
S
* Supporting Information
nanoparticle, especially for bimetallic noble metal nanostruc-
ABSTRACT: Monolayer Ru atoms covered highly
ordered porous Pd octahedra have been synthesized via
the underpotential deposition and thermodynamic control.
Shape evolution from concave nanocube to octahedron
with six hollow cavities was observed. Using aberration-
corrected high-resolution transmission electron micros-
copy and X-ray photoelectron spectroscopy, we provide
quantitative evidence to prove that only a monolayer of Ru
atoms was deposited on the surface of porous Pd
octahedra. The as-prepared monolayer Ru atoms covered
Pd nanostructures exhibited excellent catalytic property in
terms of semihydrogenation of alkynes.
tures. Herein, for the first time, we synthesize a novel bimetallic
nanostructure that is composed of monolayer Ru atoms on
highly ordered porous Pd octahedrons, which exhibit excellent
performance in catalytic semihydrogenation of alkynes.
In a typical experiment, after K2PdCl4 (19.6 mg, 0.06 mmol),
RuCl3·xH2O (12.4 mg, 0.06 mmol) and PVP (120 mg) were
dissolved in 15 mL of formaldehyde aqueous solution, the
mixture was under magnetic stirring at room temperature for 1
h. Then, the solution was transferred into an autoclave and kept
at 130 °C for 2 h. After being washed with water to remove
excess PVP, the final product was collected and characterized
by aberration-corrected high-resolution transmission electron
microscopy (HRTEM). As shown in Figures 1a and S1, the
octahedral nanostructures with a longest diameter of about 40.0
4.6 nm were observed. Interestingly, lower contrast near the
tips of octahedra clearly shows the hollow cavities with
diameter of <5 nm. A high-angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM) image
(Figure 1b) of the obtained nanostructures shows significantly
darker contrast at each tip of the octahedral and six hollow
cavities exist in a single nanoparticle. Line-scan energy-
dispersive X-ray spectroscopy (EDS) of a single octahedron
shows that the less contrast regions in the HAADF-STEM
image are due to the absence of Pd (Figure S2). Figure 1c
shows the aberration-corrected high-resolution HAADF-STEM
image of a tip of the obtained octahedron. The lattice fringes of
0.225 and 0.195 nm were corresponding to the {111} and
{200} planes of Pd, respectively. Although only Pd was found
in the octahedral nanostructures in the EDS measurement
(Figure S3), a weak binding energy peak at 280.7 eV which can
be assigned to Ru 3d5/2 of metallic Ru0 was detected by the X-
ray photoelectron spectroscopy (XPS) spectrum (Figure S4).
To further confirm the presence of Ru, the aberration-
corrected HAADF-STEM was used to investigate the obtained
octahedral nanostructures at the atomic scale (Figure 1d, e).
Note that Ru and Pd have different crystal structures, i.e.,
hexagonal close-packed (hcp) Ru with stacking sequence of
atomic layers expressed as ABAB and face-centered cubic (fcc)
Pd with atomic layers cycle between three equivalent shifted
positions marked as ABCABC. Both hcp and fcc are close-
oble metal nanostructures have been intensely inves-
Ntigated because of their fascinating properties and
applications in catalysis,1 electronics,2 surface-enhanced
Raman scattering, and sensors.3 It is well-documented that
the properties of noble metal nanostructures can be tuned by
their size, shape, and structures.4,5 Recently, bimetallic noble
metal nanostructures such as heterostructures and core−shell
structures have attracted increasing attention as they exhibit
enhanced properties and stability arising from the synergistic
effect between two metals.6−8 More importantly, only a
fractional amount of noble metal at the surface, such as few
atomic layer or monolayer noble metal formed on single crystal
metal surfaces, has significantly changed the catalytic and
optical properties of the bimetallic nanocrystals.9−11 For
example, based on the theoretical calculation, monolayer Pt
atoms on Pd (111) surface can surpass the oxygen reduction
reaction activity of pure Pt.9 Therefore, controlling the surface
of bimetallic nanostructures at the atomic level can effectively
tune catalytic and optical properties of bimetallic materials.
Moreover, much attention has been paid to the synthesis of
hollow or porous noble bimetallic nanostructures as they
provide enhanced catalytic properties.12−15 The main methods
for preparation of hollow or porous noble metal nanostructures
rely on the removal of sacrificial materials based on the
Kirkendall effect, galvanic replacements, or selective etch-
ing.12−15 Although a few bottom-up approaches were
developed for the synthesis of porous Au nanostructures with
tailorable hollow features,16 it still remains challenging to
control the formation of highly ordered porous structure in a
Received: August 24, 2015
© XXXX American Chemical Society
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DOI: 10.1021/jacs.5b08956
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Journal of the American Chemical Society
Communication
Figure 2a schematically illustrates the morphological
evolution of the ordered porous nanostructures. First, the
Figure 1. (a) TEM and (b) HAADF-STEM images of ordered porous
octahedral nanostructures. Atomic resolution aberration-corrected
HAADF-STEM images of (c) the porous area of the ordered porous
octahedral nanostructure and (d,e) the surface of octahedra. False
color was applied to enhance the contrast.
Figure 2. (a) Schematic illustration of the synthesis of ordered porous
octahedra. (b,d,f) HRTEM images and (c,e,g) STEM images of
concave nanocubes (b,c), porous truncated octahedra (d,e), and final
ordered porous octahedra (f,g).
packed structures, while fcc {111} and hcp {001} have the same
atom arrangement on their close-packed facets. On those
surfaces, the atoms arrange in a triangle fashion to maximize the
packing density (indicated by the shadow area in Figure S5a-b).
Ru {001} and Pd {111} show a moderate lattice mismatch of
3.6% on the close-packed facet, which enables the deposition of
Ru on Pd {111} surface. However, stack faults have been
observed near the surface. For example, the stacking sequence
is ABAC (from the topmost layer downward), while the second
topmost layer forms a mirror layer, i.e., a twin boundary
(Figures 1d-e, S6). More importantly, the topmost layer always
occupies the hcp sites (with respect to the substrate). This is a
very clear sign of the deposition of hcp Ru on the surface of Pd.
The hcp-to-fcc phase transition existing near the surface might
result in the formation of stack faults.8 In addition, the plane
distance between the topmost layer and the second topmost
layer is 2.53 Å, which is significantly larger than the interlayer
distance of Ru (2.15 Å, Figure S5d). This result confirms that a
monolayer rather than bilayer of Ru atoms was deposited on
the surface of Pd octahedron.
Moreover, XPS and aberration-corrected HAADF-STEM
results provide quantitative evidence that only a monolayer of
Ru atoms was deposited on Pd surface. The introduction of Ru
ions during the growth of Pd is an effective strategy to control
the morphology of Pd nanoparticle. Only solid nanoparticles
were obtained without the presence of Ru3+ (Figure S6). These
features match well with the underpotential deposition (UPD)
process, which refers to the deposition of a metal monolayer on
a foreign metal substrate at a potential much more positive than
that for deposition on the same metal surface.17,18 The UPD
process has been widely used to direct the growth of different
shaped noble metal nanocrystals like Au octahedra,19 Au
nanorods,20 Au−Pd alloy hexoctahedra,21 and Pd concave
nanostructures,22,23 although the deposited monolayer was
rarely detected.
concave nanocube with diameter of 15 nm was obtained at
room temperature for 1 h (Figure 2b-c). Then, by increasing
the reaction temperature to 130 °C, the truncated octahedron
nanoparticle with darker contrast at the center and lighter
contrast at the surface was obtained after the reaction for 15
min (Figure 2d-e). The size and shape of darker contrast part
are similar to the concave nanocube (Figure 2b), while the less
contrast part in the HRTEM and STEM image might be due to
the generated hollow cavities. As the reaction continues for 2 h,
six hollow cavities formed at the surface of concave nanocube
remained in the well-defined octahedral nanocrystals (Figure
2f-g). The shape evolution from cube to octahedron is similar
to the reported Ag, Au, and Pt nanocrystals by using PVP as a
surface-capping agent under thermodynamic control.24
To the best of our knowledge, the obtained porous Pd
octahedra covered with monolayer Ru atoms is a novel
nanostructure. The unique porous structure with trace amounts
of Ru on the surface might possess enhanced catalytic
properties. As a proof-of concept application, its catalytic
performance for the semihydrogenation of alkynes was
investigated. Traditionally, alkenes were obtained through the
semihydrogenation of alkynes using the Lindlar catalyst (Pd/
CaCO3 and Pb(OAc)2, in conjunction with quinolone).25
Unfortunately, the alkynes were easily overhydrogenated to
obtain alkanes, and the use of Pb(OAc)2 is environmentally
unfriendly.26 Figures 3a-b and S7a-b show the performance of
semihydrogenation of phenylacetylene catalyzed by using the
obtained porous Pd octahedra covered with monolayer Ru
atoms. Promisingly, 100% conversion of phenylacetylene and
90% of styrene selectivity were obtained in 4 h. In a control
experiment, the yield of phenylacetylene was only 43% when
the same amount of Pd nanoparticles was used as the catalyst
(Figure S8).
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DOI: 10.1021/jacs.5b08956
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Journal of the American Chemical Society
Communication
underpotential deposition and thermodynamic control, the Pd
octahedral nanocrystals with six hollow cavities were generated
from the Pd concave nanocubes. Stack faults near the surface of
nanocrystals were observed, and the monolayer of Ru atoms
always occupy the hcp sites. These novel bimetallic nanocryst-
als exhibited excellent catalytic activity and stereoselectivity for
the semihydrogenation of alkynes.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.5b08956.
■
S
Detailed experimental procedures; HAADF-STEM im-
ages; EDS and XPS spectrum of the monolayer of porous
Pd octahedral covered with Ru atoms nanoparticles
(PDF)
AUTHOR INFORMATION
■
Figure 3. Catalytic semihydrogenation of alkynes. (a) Conversion of
phenylacetylene and (b) selectivity of styrene catalyzed by the porous
Pd octahedra covered with monolayer Ru atoms nanoparticles and the
Corresponding Authors
*hongxun@ustc.edu.cn
*ydli@mail.tsinghua.edu.cn
Pd nanoparticles. (c) Conversion of methyl non-2-ynoate (R1
=
C6H13, R2 = CO2Me) to (d) (Z)-methyl non-2-enoate (curve 1 and 2)
and (E)-methyl non-2-enoate (curve 3 and 4) catalyzed by the porous
Pd octahedra covered with monolayer Ru atoms nanoparticles and the
Pd nanoparticles. See Supporting Information for details.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Fundamental Research Funds
for the Central Universities (WK2060190042), and the
National Natural Science Foundation of China (21571169).
H.Z. thanks the financial support from MOE under AcRF Tier
2 (ARC 26/13, no. MOE2013-T2-1-034; ARC 19/15, no.
MOE2014-T2-2-093) and AcRF Tier 1 (RGT18/13, RG5/13),
and NTU under Start-Up Grant (M4081296.070.500000) in
Singapore.
To further study the activity and stereoselectivity of the as
prepared porous Pd octahedra covered with monolayer Ru
atoms, the catalytic performance was studied by the semi-
hydrogenation of internal alkyne. The result showed that the
conversion of methyl non-2-ynoate was >99% after 2 h catalysis
reaction by using the porous Pd octahedra covered with
monolayer Ru atoms, while 56% conversion with the same
amount of Pd nanoparticles as the catalyst (Figure 3c).
Moreover, the porous Pd octahedra covered with monolayer
Ru atoms gave excellent selectivity (92%) of (Z)-methyl non-2-
enoate, and the stereoselectivity could be maintained and even
prolong the reaction time after non-2-ynoate disappeared
(Figures 3d and S7c-d). Importantly, the over-reduction
product, i.e., methyl nonanoate, was not detected during the
reaction. The stability of the porous Pd octahedra covered with
monolayer Ru atoms was conducted by a recycling test. No
obvious change was observed for the activity and selectivity
throughout four cycles (Figure S9). The TEM image of the
porous Pd octahedra covered with monolayer Ru atoms catalyst
after four catalytic cycles shows that the catalysts are still well-
dispersed and their shape is well-preserved (Figure S10).
Therefore, the monolayer Ru atoms covered porous Pd
octahedral nanoparticle is an excellent catalyst, whose specific
activity is about 5 times higher than that of the Pd catalyst (see
Figure S11 for details). The high selectivity of alkenes could be
ascribed to the synergistic effects between the monolayer Ru
atoms and the porous Pd nanocrystals. As known, ruthenium
catalysts usually form a stable catalyst−substrate complex in the
hydrogenation of alkynes,27,28 and thus the active sites on the
surface of catalysts could be blocked by the occupied Ru−
substrate complex. The role of the covered Ru atoms is similar
to the generally accepted poisoning effect of the Lindlar
catalyst.25,29
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DOI: 10.1021/jacs.5b08956
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX