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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)



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/



via kinetic control


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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Address correspondence to Yujie Xiong, [email protected]

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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

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

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


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


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


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


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


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


via kinetic control correct


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).


Nano Res.

Figure S1. Proportion of surface atoms on octahedrons with different edge lengths.


Nano Res.

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.


Nano Res.

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.


Nano Res.

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.


Nano Res.

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.


Nano Res.

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.


Nano Res.

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.


Nano Res.

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.


Nano Res.

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]


Nano Res.

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

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