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Nano Res
1
Boosting catalytic efficiency in Heck coupling reaction
by shrinking the size of Pd octahedrons down to 5 nm
via kinetic control
Ran Long§, Di Wu§, Yaping Li, Yu Bai, Chengming Wang, Li Song, and Yujie Xiong()
Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0722-1
http://www.thenanoresearch.com on January 15, 2015
© Tsinghua University Press 2015
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Nano Research
DOI 10.1007/s12274-015-0722-1
TABLE OF CONTENTS (TOC)
Boosting catalytic efficiency in Heck coupling reaction
by shrinking the size of Pd octahedrons down to 5 nm
via kinetic control
Ran Long, Di Wu, Yaping Li, Yu Bai, Chengming Wang,
Li Song, and Yujie Xiong*
Hefei National Laboratory for Physical Sciences at the
Microscale, Collaborative Innovation Center of Chemistry
for Energy Materials, School of Chemistry and Materials
Science, Laboratory of Engineering and Material Science,
and National Synchrotron Radiation Laboratory,
University of Science and Technology of China, Hefei,
Anhui 230026, P. R. China
A method has been developed to synthesize Pd octahedral nanocrystals
by manipulating the kinetics of atomic addition. The atomic addition
preferentially occurs at higher-energy surface when the atomic
concentrations are intentionally controlled very low, producing Pd
octahedrons with sizes down to 5 nm. This perfect size and facet
control enables boosting the catalytic efficiency in Heck coupling
reactions.
Provide the authors’ webside if possible.
Yujie Xiong, http://staff.ustc.edu.cn/~yjxiong/
Boosting catalytic efficiency in Heck coupling reaction
by shrinking the size of Pd octahedrons down to 5 nm
via kinetic control
Ran Long§, Di Wu§, Yaping Li, Yu Bai, Chengming Wang, Li Song, and Yujie Xiong()
Received: day month year
Revised: day month year
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
Palladium, facet control,
size control, Heck-type
coupling reaction,
catalysis
ABSTRACT
As heterogeneous catalysis essentially occurs through a process of interfacial
reactions, both surface facet and size control hold the promise for boosting catalytic
efficiency. Pd octahedral nanocrystals enclosed by {111} facets should be an ideal
geometrical shape for Heck-type coupling reactions; however, it remains
challenging to achieve the synthesis of 5-nm Pd octahedrons with relatively
uniform size distribution using the existing capping-agent techniques. In this
research article, we use palladium as a model system to perform the investigation
where kinetics of atomic addition can be precisely controlled simply using a syringe
pump. As a result, our developed method has allowed producing Pd octahedrons
down to 5 nm, offering the possibility of boosting the catalytic efficiency in the
Heck coupling reactions while reducing the usage weight of catalysts.
1. Introduction
Use of heterogeneous catalysts in organic reactions is
a promising approach to chemical and energy
conversions, as it allows for ligand-free
methodologies and facilitates easy catalyst
purification/separation and recycling.[1] The
heterogeneous catalysis essentially occurs through a
process of interfacial reactions, whose performance
thus relies on the interaction of reaction molecules
with catalyst surface. As demonstrated in many
catalytic reactions, the surface state of a catalyst plays
a crucial role in determining species adsorption and
reaction activation, and in turn, holds the key to
tailoring its activity and selectivity in catalysis.[2-16]
As such, facet control becomes a central theme of
controlled synthesis for further industrial
applications, with a focus on forming single,
controllable facets on nanocrystal surface. For
instance, it has been discovered that cubic Pd
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DOI (automatically inserted by the publisher)
Address correspondence to Yujie Xiong, [email protected]
Review Article/Research Article Please choose one
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2 Nano Res.
nanocrystals exhibit superior catalytic activity to Pd
octahedrons in Suzuki-type coupling reactions.[7]
In such a type of reactions, it turns out that the
behavior of adsorbed molecular oxygen on Pd
surface is responsible for the activity difference (i.e.,
the O2 is better activated on Pd{100} than Pd{111}).
Intuitively, we assume that Pd{111} surface may be a
promising reaction platform with desired activity for
anaerobic reactions such as Heck-type coupling
reactions.
In addition to the facets, particle size is another
highly important parameter to catalytic activity. It
is well recognized that the surface atoms will become
more dominant in the catalytic reactions as the size of
catalytic particles shrinks.[17] Thus reducing the
particle size of nanocrystals represents a versatile
route to boost catalytic efficiency and improve atomic
economy.[18, 19] Taken together, precise control
over the surface facets and sizes of nanocrystals
should be fundamentally important to development
of heterogeneous catalysts.
In recent years, use of capping agents has been
a major tool for manipulating surface facets, in both
direct synthetic scheme and seed-mediated growth
system.[20-23] The function of capping agents
mainly works through modifying surface energies of
various facets in terms of thermodynamics. For
instance, citric acid has been proven a valid agent for
promoting {111} facets of metallic nanocrystals to
yield Pd octahedrons.[22-24] According to the
estimate in Figure S1, the surface atoms of
octahedrons can be dramatically increased from 14%
to 33% of total atoms when their edge lengths shrink
from 12 nm to 5 nm. However, it remains
challenging to achieve the synthesis of 5-nm Pd
octahedrons with relatively uniform size distribution
using the existing capping-agent techniques. One of
common used methods for size control is to stop
reactions at early stage of nanocrystal growth.[25]
However, this method cannot make full use of raw
chemicals, which will be a disadvantage for
industrial applications. More importantly,
nanocrystals often take a very different shape at their
early growth stage. Only cuboctahedral
nanocrystals can be obtained if the synthesis of Pd
octahedrons is terminated too early.[25]
In this research article, we demonstrate that Pd
octahedral nanocrystals can be synthesized by
manipulating the kinetics of atomic addition simply
using a syringe pump in a two-stage feeding process,
with their sizes precisely narrowed down to the 5-nm
regime with high size consistency. The {111} facets
formed at the surface of octahedrons would enable to
investigating facet-dependent catalytic effects, with
well-developed {100}-bound Pd nanocubes as a
reference. The size shrinkage allows us to
maximizing surface atoms with high activity for the
Heck-type coupling reactions. This work not only
advances our understanding on kinetic control of
atomic addition in nanocrystal synthesis, but also
offers a powerful means for controlling the shape
and size of Pd octahedrons by simply adjusting the
feeding rates of metal precursor.
2. Experimental
2.1 Aqueous synthesis: In a typical synthesis, 0.1050
g of poly(vinyl pyrrolidone) (PVP, M.W.=55,000,
Sigma-Aldrich, 856568-100g), 0.0600 g of citric acid
(Sigma-Aldrich, 251275-100g) and 0.0600 g of
L-ascorbic acid (Sigma-Aldrich, A0278-25g) were
dissolved in 8 mL of deionized water at room
temperature. The solution was placed in a 3-neck
flask (equipped with a reflux condenser and a
magnetic Teflon-coated stirring bar) and heated in air
at 120 °C for 5 min. Meanwhile, 0.0650 g of
potassium palladium(II) chloride (K2PdCl4, Aladdin,
1098844-1g) was dissolved in 3 mL of deionized
water at room temperature. The Pd stock solution
was then injected into the flask through a syringe
pump at a specific rate (360 mL/h, 20 mL/h, 10 mL/h
or 5 mL/h). Heating of the reaction at 120 °C was
continued in air for 3 h after injection of Pd stocking
solution. A set of samples were taken over the
course of each synthesis (both feeding stages and
post-feeding stages) with a glass pipet. The samples
were washed with acetone and then with deionized
water several times to remove most of the PVP by
centrifugation. The as-obtained samples were then
characterized by transmission electron microscopy
(TEM) and high-resolution TEM (HRTEM).
2.2 Two-stage feeding scheme: The experimental
procedure is similar to that in the aqueous synthesis
above, except using different feeding rates in two
separate stages. In a typical synthesis, 0.1050 g of
PVP (M.W.=55,000), 0.0600 g of citric acid and 0.0600
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3 Nano Res.
g of L-ascorbic acid were dissolved in 8 mL of
deionized water at room temperature. The solution
was placed in a 3-neck flask (equipped with a reflux
condenser and a magnetic Teflon-coated stirring bar)
and heated in air at 120 °C for 5 min. Meanwhile,
0.0650 g of K2PdCl4 was dissolved in 3 mL of
deionized water at room temperature. Part of the
Pd stock solution (0.5 mL or 1.0 mL) was then
injected into the flask through a syringe pump at a
high rate (360 mL/h) first, followed by feeding the
rest stock solution (2.5 mL or 2.0 mL) at a low rate (5
mL/h). Heating of the reaction at 120 °C was
continued in air for 3 h.
2.3 Heck coupling reactions: All the reactions were
performed in a stirred, glass tube (25 ml). Typically,
0.25-mmol iodobenzene, 0.35-mmol ethyl acrylate,
1-mmol K2CO3 and certain amount of Pd catalysts
were suspended in 5-mL DMF. Then the air in the
glass tube was purged and replaced by Ar gas three
times and then sealed for further reaction. The
reactions were typically carried out at 120 ̊C for 9
hours. For the identification and analysis of the
coupling products, a gas chromatography-mass
spectrometry (GC-MS, 7890A and 5975C, Agilent)
was employed.
3. Results and discussion
Prior to our investigations, we have to validate
whether the Pd{111} facets possess higher activity
than the Pd{100} as predicted. We have employed
the Heck coupling between iodobenzene and ethyl
acrylate as a model reaction to prove our concept
(Scheme 1), with Pd{100}-nanocubes and
Pd{111}-octahedrons as catalysts. In the beginning
of this investigation, the real active sites (plane atoms
or edge atoms) for Heck-type reactions should be
first determined. To identify the catalytic sites for
this reaction,[21] three different sizes of Pd
nanocubes (8 nm, 15 nm and 26 nm in edge length,
see Figure S2a-c) are used as catalysts in the Heck
reaction.[19] When the molar ratios of surface Pd
atoms to reactants are set as 0.7‰, the yields for
Heck reaction are identical (53-56%) using the
nanocubes at different sizes as catalysts (see Table 1).
It indicates that the number of surface atoms (i.e., the
atoms located at the flat surface) is the most
important parameter to determine the catalytic
activities of catalysts in Heck-type coupling reaction.
To assess the facet effect, 8-nm Pd octahedrons
covered by {111} facets that have been synthesized by
following the previously reported protocol (Figure
S2d) are used in the catalytic Heck coupling reaction
for comparison. As shown in Table 1, the catalytic
activity of {111} facets in the Heck reaction is
significantly higher than that of {100} facets as
expected (84% versus 53% in coupling yield at the
same catalyst concentration in terms of surface
atoms). Upon recognizing the favorable role of
Pd{111} in the Heck reaction, we can conclude that
Pd octahedrons with minimized particle sizes should
be ideal catalysts for the Heck-type coupling reaction
depicted in Scheme 1.
Scheme 1 Model Heck-type reaction and Pd nanocrystal
catalysts used in the present study.
Then the big challenge raised by this material
demand is how to achieve the size control along with
the facet evolution on metal nanocrystals. For the
facet evolution, we can find some clues about this
aspect in literature: silver nanocubes evolved into
cuboctahedrons and then octahedrons by adding
silver atoms onto nanocrystal surface in mediation of
foreign metals.[26] Although this example
highlights the possibility of forming {111} facets
through the nanocrystal growth, it does not allow us
to tightly control the particle size to the desired
regime as the octahedral shape only arises at the final
growth stage. To address this grand challenge, we
have to find a way to precisely manipulating the
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4 Nano Res.
growth mode of metal nanocrystals within the
timeframe of early nucleation and growth. In
efforts to maneuver the nucleation and growth,
kinetics of atomic addition is a critical parameter to
the competition between heterogeneous growth and
homogeneous nucleation. In the previous
shape-controlled synthesis studies, somehow the
importance of kinetics of atomic addition in
nanocrystal growth is commonly neglected. In fact,
it has been lack of a simple reaction system to fully
investigate how kinetic control over atomic addition
can impact on the competing growth of various
surface facets beyond the control of thermodynamics.
In the present work, we use palladium as a model
system to perform the investigation on the kinetics of
atomic addition that can be precisely controlled
simply using a syringe pump. In the palladium
system, majority of products is single-crystal in the
region governed by thermodynamics, which can
largely simplify the influence from twinned
structures (i.e., strain energy caused by twin defects).
In the synthesis, we employ a simple reaction
system which includes K2PdCl4 (Pd precursor),
poly(vinyl pyrrolidone) (PVP, stabilizer), citric acid
and ascorbic acid (co-reductant) in aqueous media,
without any other agents involved. Such a
reduction of Pd precursor quickly generates 6-nm
single-crystal nanoparticles at a high feeding rate of
360 mL/h (see Figure 1a). The feeding rate and
timeframe are precisely and automatically controlled
by a syringe pump. As the reaction is under
thermodynamic control, single-crystal nanoparticles
are formed within the context of Wulff’s theorem,
which attempts to minimize the total interfacial free
energy of a system within a given volume.[27] As a
result, they tend to exist as cuboctahedrons enclosed
by a mix of {111} and {100} facets, as indicated by
high-resolution transmission electron microscopy
(HRTEM, Figure 1b and 1c). This shape has a nearly
spherical profile to minimize the total interfacial free
energy. However, when the feeding rate is
controlled down to 5 mL/h, the growth of
nanocrystals can be turned into a different mode.
Figure 1d shows a typical TEM image of products
which mainly consist of Pd octahedrons. HRTEM
characterizations (Figure 1e and 1f) verify the
majority of {111} facets on surface although a very
small portion of {100} facets can be observed. Based
on the statistics on over 100 particles, about 19%
particles have slightly truncated corners towards the
cuboctahedral shape. Note that Cl- and O2 can
selectively dissolve twinned Pd nanostructures.[27,
28] Since the Pd precursor used in this work –
K2PdCl4 contains a significant amount of Cl-, the
twinned seeds can be easily removed by the Cl-/O2
etchants. As shown in Figure S3, a sample from the
early stage of synthesis contains only one or two
twinned nanostructures in about one hundred
nanoparticles. As the reaction proceeds, the
twinned nanoparticles are gradually dissolved,
leaving high-purity single crystals in the product.
Figure 1 TEM images of the samples prepared from an
aqueous synthesis at different feeding rates: (a-c) 360 mL/h and
(d-f) 5 mL/h. (b, c) and (e, f) show HRTEM images of the
samples in (a) and (b), respectively.
To elucidate the formation mechanism, we
have collected samples from different feeding stages
(see Figure S3), showing that the nanocrystals take a
cuboctahedral shape in very early stage of feeding
(i.e., early phase of growth). Beyond the completion
of feeding stock solution, we perform
time-dependent studies and confirm that the
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5 Nano Res.
octahedral shape of nanocrystals is well maintained
(see Figure S4), indicating that the octahedrons have
been formed at the feeding stage. It reveals that Pd
cuboctahedrons evolve into an octahedral shape
through slow atomic addition onto specific facets
while continuously feeding stock solution. Given
the structure difference between cuboctahedrons and
octahedrons, we can conclude that the majority of
{111} facets in final products is mainly ascribed to the
selective atomic addition to {100} and {110} facets
controlled by slow feeding during the nanocrystal
growth.
Previously the effect of capping agents has
been considered as a major factor in promoting {111}
facets on nanocrystal surface.[22, 23] It happens
that citric acid is used as a co-reductant in our
synthetic system. To exclude the possibility of citric
acid in promoting {111} facets, we have performed
the control experiment in absence of citric acid which
can still yield {111}-bounded octahedrons (see Figure
S5). Thus we propose that kinetics of atomic
addition is the main factor resulting in the alteration
of ratios between various facets. More feeding
rate-dependent experiments (see Figure S6) further
confirm that the formation of {111}-bound
octahedrons is driven by the kinetics. It is
worthwhile mentioning that the strategy of
controlling surface facets by slow feeding can be
applied to various reaction systems including polyol
process – a well-developed method for producing Pd
cuboctahedrons.[28] Similarly to the aqueous
synthesis, lower feeding rate can effectively promote
the formation of Pd octahedrons in the polyol
reduction (see Figure S7). Since this system does
not involve citric acid or other agents that might
promote {111} facets, the generic feature here further
demonstrates that the facet control works through
the manipulation of atomic addition kinetics rather
than by capping effects.
Overall, the selective atomic addition to {100}
and {110} facets during slow feeding should be
ascribed to the different surface energies of various
facets. For an fcc structure with a lattice constant of
a, the surface energies of the low-index
crystallographic facets that typically encase
nanocrystals can be estimated as: γ{100} = 4(ε/a2),
γ{110} = 4.24(ε/a2), and γ{111} = 3.36(ε/a2), resulting in
the energetic sequence of γ{111} < γ{100} < γ{110}.[27]
This sequence implies that Pd atoms should be
preferentially added to the {100} and {110} facets,
when atomic concentration is maintained extremely
low to control the kinetics of atomic addition. The
preferentially atomic addition will lead to the
enlargement of {111} facets and eventually the
formation of {111} surface on nanocrystals (see the
schematics in Figure 2a). Certainly in the case of
high atomic concentration, the chances of newly
formed atoms reaching various facets should be
almost equivalent so that thermodynamically favored
cuboctahedrons can be obtained.
Figure 2 (a) Schematics illustrating the growth of Pd
nanocrystals at different feeding rates. TEM images of the
samples prepared with a two-stage feeding process: (b) 0.5 mL of
stock solution at 360 mL/h followed by 2.5 mL of stock solution
at 5 mL/h; (c) 1.0 mL at 360 mL/h and then 2.0 mL at 5 mL/h.
Size distribution histograms for the samples prepared through (d)
constant 3.0-mL feeding at 5 mL/h (i.e., the sample in Figure 1d);
(e) 0.5-mL feeding at 360 mL/h and then 2.5-mL feeding at 5
mL/h (i.e., the sample in Figure 2b); (f) 1.0-mL feeding at 360
mL/h and then 2.0-mL feeding at 5 mL/h (i.e., the sample in
Figure 2c). The average sizes and deviations of (d), (e) and (f)
are 7.39±1.66 nm, 4.92±0.68 nm and 4.51±0.32 nm, respectively.
Although slow feeding can promote the
production of octahedrons, the size distribution of
the obtained nanocrystals is relatively large mainly
due to the successive nucleation in early stage when
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6 Nano Res.
stock solution is continuously introduced into the
reaction. On the other hand, the portion of
octahedral structures with sharp corners in the
sample needs to be improved. To improve the size
distribution and shape consistency, we have
developed a two-stage feeding scheme (i.e., fast
feeding first for homogenous nucleation, followed by
slow feeding for growth into {111}-enclosed
octahedrons). This scheme is analogous to the
seeding process with cuboctahedral nanoparticles as
seeds, but uniquely enables the generation of tiny
cuboctahedral seeds in early stages, offering the
opportunity of shrinking the sizes of final products.
The seeding process generally uses nanocrystals with
relatively large sizes as seeds due to the difficulty of
separating tiny particles from solution. In our
two-stage scheme, it skips the step of taking the
formed seeds out of solution so as not to suffer from
this limitation. As such, we can take advantage of
facet control through manipulating the kinetic of
atomic addition, and at the same time, control the
particle size to narrow distribution with fast
nucleation. Figure 2b and 2c show typical TEM
images of two samples prepared with different ratios
between two feeding stages, confirming that the size
distribution of obtained octahedrons has been greatly
improved (see size comparison in Figure 2d-f). This
scheme cannot only improve the size distribution of
final products, but also demonstrate that octahedrons
are formed by kinetic control of atomic addition
indeed. Notably, over 95% nanocrystals possess
sharp corners in both two samples, showing high
shape consistency. It is worth mentioning that the
synthetic system of Pd octahedrons can be scaled up
to increase the amount of products (see Figure S8).
Upon achieving the facet control, we are now
in a position to assess the performance of Pd
nanocrystals in the proposed Heck coupling reaction.
In the assessment, the usage dose of catalysts is
maintained constant for all the catalysts in terms of
surface atoms (molsurface = 0.7‰). As displayed in
Figure 3, the catalytic activity of Pd nanocubes is
significantly lower than that of octahedrons as
indicated by turnover numbers (TONs) in terms of
both surface atoms (TONsurface) and total atoms
(TONtotal), which makes the {111} facets become more
suitable to Heck-type reactions. Notably, TONsurface
is identical with the nanocrystals at different sizes,
but TONtotal significantly increases with the particle
size shrinking. This observation can be well
understood from the illustration in Figure S1: the
ratio of surface atoms to total atoms increases from
22% to 36%, as the size of octahedrons is reduced
from 7.4 nm to 4.5 nm. The large portion of surface
atoms in turn enables a great improvement in
catalyst atomic economy.[17] Even when the usage
dose of octahedrons is reduced half (molsurface =
0.35‰), the yield in the Heck coupling reaction still
can reach >99% with a TONsurface as high as 2.85×105
(see Table 1).
Figure 3 Turnover numbers (TONs) of Pd octahedrons and
nanocubes at different sizes in terms of (a) surface atoms and (b)
total atoms.
Furthermore, our stability measurements
reveal that the Pd octahedrons exhibit
well-maintained catalytic activity after the
suspension storage for different period of time up to
35 days (see Table S1), which is extremely beneficial
to practical applications.[29] To further prove the
performance stability of our samples, we have
carried out catalytic reactions by recycling the
catalysts in 3 cycles. In order to verify the reaction
repeatability, the recycling reactions have been taken
by two replicate samples under the same
experimental conditions. Both two samples show
stable performance in the 3 runs of catalytic reactions
with yields above 99% (see Table S2). This result
suggests that the Pd octahedrons synthesized by our
method have excellent performance stability in
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7 Nano Res.
catalyst recycling tests. Note that our synthesized
Pd octahedrons exhibit slightly higher catalytic yield
than the ones in literature (>99% versus 84%) despite
their comparable average size (i.e., 7~8 nm), most
likely because our sample contains a number of small
nanocrystals due to the relatively broad size
distribution in the one-step feeding scheme.
Table 1 Heck-type coupling reactions in Scheme 1 catalyzed
by Pd nanocubes and octahedrons at the same catalyst
concentration in terms of surface atoms (molsurface=0.7‰) except
for the Reaction 8. The catalyst concentration for the Reaction
8 is half those for the Reaction 1-7 (molsurface=0.35‰).
Reaction conditions: iodobenzene (0.25 mmol), ethyl acrylate
(0.35 mmol), K2CO3 (1 mmol), DMF (5 mL), 120 C, 9 h, Ar
ambience.
Catalyst Pd average size
(nm)
moltotal
(%) Yield (%)a)
1 None - - -
2 Nanocubes
(Figure S2a) 8 0.5 53
3 Nanocubes
(Figure S2b) 15 1 56
4 Nanocubes
(Figure S2c) 26 1.7 53
5 Octahedrons
(Figure S2d) 8 0.4 84
6 Octahedrons
(Figure 1d) 7 0.4 >99
7 Octahedrons
(Figure 2b) 5 0.2 >99
8 Octahedrons
(Figure 2b) 5 0.1 >99
a) Yield for the coupling product based on iodobenzene
determined by GC-MS.
4. Conclusion
In summary, we have developed a method for
synthesizing Pd octahedral nanocrystals by
manipulating the kinetics of atomic addition. {100}
and {110} surface facets of cuboctahedral seeds have
higher surface energies so that they can preferentially
provide sites for the addition of newly formed Pd
atoms when the atomic concentrations are
intentionally controlled very low. This selective
atomic addition leaves {111} facets on the resulted
octahedral nanocrystals. The synthetic strategy has
been generically applied to and verified in multiple
reaction systems and schemes. Based on the kinetic
control of atomic addition, a two-stage feeding
scheme is designed and applied to the synthesis
system to improve the size distribution within the
regime of 4.92±0.68 nm.
Based on the optimized reaction conditions, we
next set out to investigate the substrate scope (see
Table 2). Both the examinations of substrates with
electron-donating groups and electron-withdrawing
groups show high reaction activities (yield > 99%).
Table 2 Substrate scope. Reaction conditions: 1 (0.25 mmol),
2 (0.35 mmol), K2CO3 (1 mmol), DMF (5 mL), 120 C, 9 h, Ar
ambience.
In the past, the importance of atomic addition
kinetics had been overlooked while capping agents
were used as a major tool in facet control of metallic
nanocrystals. It is anticipated that the present work
will shed light on consideration of growth kinetics
when designing a controlled synthesis, regardless of
whether capping agents are used to modify surface
energies in favor of thermodynamics, in both direct
synthesis from metal salts and seeding system. The
facet control enabled by this work offers a platform
to investigate facet-dependent catalytic effects, which
will in turn provide information for designing
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8 Nano Res.
optimal nanocatalysts for various applications. As a
proof-of-concept demonstration, the synthesis of Pd
octahedrons down to 5 nm reported here offers the
possibility of boosting the catalytic efficiency in the
Heck coupling reactions while reducing the usage
weight of catalysts.
Acknowledgements
This work was financially supported by the NSFC
(No. 21101145), Recruitment Program of Global
Experts, CAS Hundred Talent Program,
Fundamental Research Funds for the Central
Universities (No. WK2060190025, WK2060190037,
WK2310000035), and China Postdoctoral Science
Foundation (2014M560514).
Electronic Supplementary Material: Supplementary
material is available in the online version of this
article at http://dx.doi.org/10.1007/s12274-***-****-* References
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Electronic Supplementary Material
Boosting catalytic efficiency in heck coupling reaction
by shrinking the size of Pd octahedrons down to 5 nm
via kinetic control correct
Ran Long§, Di Wu§, Yaping Li, Yu Bai, Chengming Wang, Li Song, and Yujie Xiong()
Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)
EXPERIMENTAL PROCEDURE:
Polyol process: In a typical synthesis, 5 mL of ethylene glycol (EG, Aladdin, 1095698-500mL) was placed in a
3-neck flask (equipped with a reflux condenser and a magnetic Teflon-coated stirring bar) and heated in air at
110 C for 1 h to boil off trace amounts of water. Meanwhile, 0.1535 g of K2PdCl4 and 0.0800 g of PVP
(M.W.=55,000) were separately dissolved in 3 mL of EG at room temperature. These two solutions were then
injected simultaneously into the flask rapidly or through a syringe pump at a specific rate (45 mL/h or 5 mL/h).
Heating of the reaction at 110 C was continued in air for 3 h.
TEM characterizations: A drop of the aqueous suspension of particles was placed on a piece of carbon-coated
copper grid and dried under ambient conditions. TEM images were taken on a JEOL JEM-2010 LaB6
high-resolution transmission electron microscope operated at 200 kV. HRTEM images were taken on a JEOL
JEM-2100F field-emission high-resolution transmission electron microscope operated at 200 kV.
Concentration measurements: Pd nanoparticles were dissolved with a mixture of HCl and HNO3 (3:1, volume
ratio) which was then diluted with 1% HNO3. The concentration of palladium was then measured with a
Thermo Scientific PlasmaQuad 3 inductively-coupled plasma mass spectrometry (ICP-MS).
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Figure S1. Proportion of surface atoms on octahedrons with different edge lengths.
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Figure S2. TEM images of (a) 8-nm Pd nanocubes, (b) 15-nm Pd nanocubes, (c) 26-nm Pd nanocubes, and (d) 8-nm
Pd octahedrons. The sizes denoted here mean the edge lengths of Pd nanocrystals.
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Figure S3. TEM images of the samples taken from the synthesis in Figure 1d at different feeding stages: (a) upon
adding 1.5 mL of stock solution; (b) upon adding 2.5 mL of stock solution. The images show that cuboctahedrons
were generated at early stage of feeding and then grew into octahedrons with continuous feeding.
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Figure S4. TEM images of the samples taken from the synthesis in Figure 1d for different reaction time after
completely feeding stock solution: (a) 0 h; (b) 3 h; (c) 18 h; (d) 27 h. The results show no obvious morphological
change during the post-feeding reaction period, indicating that the octahedrons were mainly formed through a kinetic
control of atomic addition at the feeding stage.
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Figure S5. TEM image of prepared under the same condition as that in Figure 1d, except the absence of citric acid.
It shows that octahedrons can be obtained without the addition of citric acid, implying that the formation of {111}
facets on nanocrystal surface is not ascribed to the capping effect from citric acid.
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Figure S6. TEM images of the samples prepared under the same condition as that in Figure 1d, except the use of
different feeding rates: (a) 20 mL/h; (b) 10 mL/h. The experiments show that the formation of octahedrons was
gradually favored when decreasing the feeding rates, verifying that the shape of nanocrystals was controlled by the
kinetics of atomic addition.
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Figure S7. TEM images of the samples prepared from a polyol process by adjusting feeding rates: (a) quick injection;
(b) 45 mL/h; (c) 5 mL/h. (d) shows a HRTEM image of the octahedron in sample (c). This series of experiments
confirm that it is a generic method to control the facets of nanocrystals by manipulating the kinetics of atomic
addition, and exclude the possibility of capping agents (e.g., citric acid) in promoting {111} facets.
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Figure S8. TEM image of the sample prepared under the same condition as that in Figure 2d, except that the reaction
system was scaled up by 4 times.
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Table S1. Heck-type coupling reactions in Scheme 1 that were catalyzed by the Pd octahedrons upon the storage
for varied time period.
Sample storage time (days) Yield (%)
5 >99
10 >99
20 >99
35 >99
Address correspondence to Yujie Xiong, [email protected]
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Table S2. Heck-type coupling reactions in Scheme 1 that were catalyzed by the Pd octahedrons recycled from the
last run of reactions. In order to verify the reaction repeatability, the recycling reactions were taken by two
replicate samples under the same experimental conditions.
Number of cycles Yield of sample 1(%) Yield of sample 2(%)
1 >99 >99
2 >99 >99
3 >99 >99