Compositionally controlled self-assembly of hierarchical Pd-Ni bimetallic chains

Article abstract of DOI:10.1002/ejic.201402173

Magnetic Pd-Ni bimetals with various compositions were synthesized by a one-pot reaction, in which chain-like Pd-Ni bimetals formed through the self-assembly of Pd-Ni nanoparticles. The self-assembly behaviour of the Pd-Ni bimetals gradually changes with

Full text of DOI:10.1002/ejic.201402173

FULL PAPER
DOI:10.1002/ejic.201402173
Compositionally Controlled Self-Assembly of Hierarchical
Pd–Ni Bimetallic Chains
[
a]
[b]
[a]
[a]
Peiyu Jin, Xin Liang,* Yeheng He, Sijia Liu, and
[
a]
Xiaolin Zhu
Keywords: Self-assembly / Palladium / Nickel / Nanoparticles / Heterogeneous catalysis
Magnetic Pd–Ni bimetals with various compositions were
synthesized by a one-pot reaction, in which chain-like Pd–Ni
bimetals formed through the self-assembly of Pd–Ni nano-
particles. The self-assembly behaviour of the Pd–Ni bimetals
gradually changes with the changes in composition. The Pd–
Ni nanoparticles form simple chains at lower Pd content,
while the Pd–Ni bimetals take on a beads-on-string structure
composed of sphere-shaped Pd–Ni-nanoparticle assemblies
at higher Pd content. Magnetic hysteresis loops show that
Ni-rich assemblies have a higher saturation magnetization
than Pd-rich assemblies. The formation mechanism for these
Pd–Ni assemblies is proposed, in which the self-assembly be-
haviours are controlled by the competition of the surfactant
interaction and the magnetic dipole interaction. The catalytic
activity and selectivity of the hydrogenation of acetophenone
over these Pd–Ni bimetals depend on their compositions.
Introduction
opportunity to obtain novel catalysts with enhanced selec-
tivity, activity and stability. Various bimetals with the
[
7]
The self-assembly of small building blocks into hierarchi- combination of 3d transition metals (e.g. Fe, Co and Ni)
cal nano- or microstructures is an efficient approach for along with nearly magnetic 4d metals (e.g. Rh, Pd and Ag)
[
8]
constructing advanced artificial materials with the desired have been extensively investigated, in which the Pd–Ni bi-
structures and functionalities.[1] Generally, the self-assembly metal is one of the most promising materials with signifi-
of the building blocks can be driven by the noncovalent cant potential in catalysis. It has been realized that Pd–Ni
interactions between components, such as hydrogen bond- bimetals are magnetic materials and their properties are
ing interactions, charge-transfer interactions, van der Waals strongly composition dependent. Thus, Pd–Ni bimetallic
forces, and electric/magnetic dipole interactions.[ The con- systems could serve as an ideal system to investigate the
trol of the interaction between components is a powerful self-assembly behaviours under tunable non-covalent inter-
strategy for controlling the assembling modes of the build- actions by controlling their compositions.
2]
ing blocks. For example, the assemblies tend to reach the
Herein, an example of compositionally controlled self-
equilibrium state under weak non-covalent interactions and assembly has been demonstrated in which hierarchical Pd–
form a non-equilibrium system under strong non-covalent Ni bimetallic chains with different assembling modes have
[
3]
interactions. The interaction is significantly influenced by been achieved. This paper shows how the composition fac-
[
4]
the properties of the building blocks, such as the size,
shape, magnetic properties and the capped organics.
tor governs the structural, morphological and magnetic
properties of Pd–Ni bimetals, the non-covalent interactions
[
5]
[6]
Thus, investigations on the effect of factors on the self-as- between components, and the self-assembly of the hierar-
sembly behaviours of the building blocks lead to the design chical chains. The catalytic properties of these Pd–Ni bime-
and synthesis of complicated assemblies by modulating the tallic assemblies were systematically investigated by the hy-
building blocks.
drogenation of the acetophenone (ACP) reaction, which
Bimetallic nanocrystals, which are distinct from those of shows that the catalytic hydrogenation performance of these
their parent metals, show a composition-dependent surface Pd–Ni assemblies is strongly dependent on their composi-
structure and atomic segregation behaviour, and offer the tions.
[
[
a] Department of Chemical Engineering, Beijing University of
Chemical Technology,
Results and Discussion
Beijing 100029, P. R. China
b] State Key Laboratory of Chemical Resource Engineering,
Beijing University of Chemical Technology,
Pd–Ni bimetallic assemblies with tunable compositions
were obtained by the one-pot reduction of Pd(Ac) and
1
00029 Beijing, P. R. China
2
E-mail: liangxin@mail.buct.edu.cn
Ni(Ac) ·4H O in the presence of oleylamine and tetrabutyl
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402173.
2
2
ammonium bromide (TBAB), in which the samples derived
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Figure 1. (a) X-ray diffraction patterns of Pd–Ni bimetallic assemblies,and (b) the enlarged parts within the range from 2θ values 38° to
3°.
4
2
+
2+
as Pd /Ni in molar ratios of 1:3, 2:3, 1:1 and 3:1 were more hierarchical beads-on-string chain-like structure. For
denoted as Pd Ni , Pd Ni , Pd Ni and Pd Ni , Pd Ni , the sample also takes on the beads-on-string
2
5
75
40 60
50 50
75 25
75 25
respectively. The X-ray diffraction (XRD) patterns of the structure, while the secondary spheres became larger, at the
prepared series of Pd–Ni alloys are demonstrated in Fig- rough range of 200–400 nm, see Figure 2 (d). The unique
ure 1. For Pd Ni and Pd Ni , the X-ray diffractive and complex structures of these chain-like Pd–Ni bimetals
5
0
50
75 25
peaks can be assigned to {111}, {200} and {220} diffrac- were further confirmed by SEM characterization (Fig-
tions of a pure phase of face-centered cubic crystal struc- ure 3). The compositionally controlled morphology evol-
tures, in which the peaks are located between the corre- ution from the randomly assembled chains of the Ni-rich
sponding standard peaks of pure Pd (JCPD-65-6174) and samples to the hierarchical beads-on-string chain-like struc-
Ni (JCPD-04-0850), arising from the formation of Pd–Ni tures of the Pd-rich samples can be observed. Obviously,
alloys. The peaks shift from the palladium standard peaks the compositions control the phase, morphology and the
to the nickel standard peaks with the increase of nickel con- self-assembly behaviour of the as-prepared Pd–Ni bimetals.
tent from Pd Ni to Pd Ni , and no other peaks were The accurate compositions of the as-prepared Pd–Ni bime-
5
0
50
75 25
observed in the XRD patterns, revealing that only Pd–Ni tallic chains were determined by SEM-EDX spectroscopy.
alloys formed in these cases. For Ni-rich samples (Pd Ni The results show that Pd/Ni molar ratios in the Pd–Ni
and Pd Ni ), two sets of X-ray diffractive peaks can be chains are similar to the input Pd/Ni molar ratios of reac-
40
60
2
5
75
observed from the XRD patterns, which can be attributed
to the cubic phase of Pd and Ni crystal structures. The dif-
fractive peaks at 2θ values of 44.5°, 51.8° and 76.4° are well
assigned to the nickel standard peaks. With the increase of
nickel content from Pd Ni to Pd Ni , the intensity of
4
0
60
25 75
these Ni peaks gradually strengthens, and the shift of the
Pd111 peaks weakens. The reinforced cubic nickel structures
accompanied with the weakened intermetallic alloy struc-
ture resulted in a mixed heterogeneous structure for the Ni-
rich samples.
The morphologies and size of the as-prepared Pd–Ni bi-
metals were observed by TEM and SEM images. TEM
images in Figure 2 show that all the Pd–Ni bimetals display
hierarchical chain-like structures that are composed of
small primary Pd and Ni nanoparticles. The composition
of these Pd–Ni bimetals plays a vital role in controlling the
self-assembly modes of these Pd–Ni chains. For the Ni-rich
samples, the Pd–Ni short chains were formed through ran-
dom assembly of Pd–Ni nanoparticles with the particle size
of 15–20 nm. The assembling modes of these Pd–Ni chains
become more hierarchical along with the increase of palla-
dium content. For Pd Ni – see Figure 2 (c) – the small
5
0
50
primary Pd–Ni alloy nanoparticles assemble into bigger
spheres with a diameter of about 150 nm, and then, these
Figure 2. TEM images accompanied by the different magnifications
lower left inset) of the as-prepared (a) Pd25Ni75, (b) Pd40Ni60
(
,
secondary spheres link together to shape the unique and (c) Pd50Ni50 and (d) Pd75Ni25
.
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tants (Table S1, Supporting Information). The EDX spectra als, such as FePt and NiCo magnetic materials.[10] To
are shown in Figure S2.
understand the underlying self-assembly mechanism of
these Pd–Ni hierarchical chain-like assemblies, the forma-
tion and assembly processes are carefully explored by using
Fourier transform infrared spectroscopy (FTIR) and vi-
bration sample magnetometry (VSM).
Figure 4 shows FTIR spectra of the prepared Pd–Ni bi-
metallic powders with various compositions. The spectra
show characteristic peaks of the absorbed oleylamine
group: the bands at 2920 and 2852 cm–1 can be assigned to
the symmetric and asymmetric methylene stretching modes,
the peaks at around 1628 cm–1 can be assigned to the
ν(C=C) stretching modes of the oleylamine molecules and
the wide absorption band around 3430 cm–1 is from the
ν(N–H) stretching mode. The strong absorption bands at
–1
around 1458, 1445 and 1404 cm can be attributed to
[
11]
tetrabutyl ammonium cations.
FTIR spectra confirm
that both oleyl group and tetrabutyl ammonium cations are
coated on the surface of the Pd–Ni bimetals.
Figure 5 shows the field-dependent magnetization (M–
H) curves of these Pd–Ni bimetals with different composi-
tions recorded at 300 K. The M–H curves exhibit clear hys-
teresis loops for all the samples, demonstrating the ferro-
Figure 3. SEM images of prepared (a) Pd25Ni75, (b) Pd40Ni60
,
(c) Pd50Ni50 and (d) Pd75Ni25.
Generally, the possible non-covalent interactions be- magnetic behaviours of these Pd–Ni bimetallic materials.
tween the building blocks include hydrogen bonding inter- The particle sizes of the as-prepared Pd–Ni nanoparticles
actions, charge-transfer interactions, Van der Waals forces (15–20 nm) are in the range of a single magnetic domain,
and electric/magnetic dipole interactions. For the as-pre- thus, the primary Pd–Ni nanoparticles can be considered as
pared Pd–Ni bimetals, oleylamine and cationic surfactant mini-magnets with substantial magnetic moments. Because
TBAB were applied as both stabilizer agent and surfactant, of the magnetic dipole–dipole interaction of these mini-
[
12]
which are significant for the control of the nucleation and magnets,
these Pd–Ni nanoparticles have a tendency to
growth process of the Pd–Ni nanoparticles. Comparison ex- attract each other to form assemblies. Linear chains will
periments also show that only irregular nanoparticles form form along the magnetic anisotropy direction if the mag-
in the absence of TBAB (Figure S3 in SI). Studies have netic dipolar interactions outweigh the thermal motion.
demonstrated that interactions between the surfactants Otherwise, the direction of these mini-magnets would be
could induce the self-assembly of the nanoparticles.[9] It can randomly distributed because of the strong thermal motion.
be concluded that the surfactant in the synthetic systems is Thus, the strength of the magnetism has a great influence
responsible for the self-assembly behaviour of these Pd–Ni on the self-assembly behaviour of these Pd–Ni nanopar-
bimetals. On the other hand, the magnetic properties of Pd– ticles. The stronger magnetism favours the self-assembly
Ni are also crucial for the formation of such chain-like tendency of the nanoparticles because of the higher mag-
assemblies. Many nanoparticle chains have been demon- netic dipolar interaction. The magnetic measurements show
strated in the ferromagnetic or superparamagnetic materi- that the saturation magnetization values (Ms) are 26.92,
Figure 4. FTIR spectra of the prepared Pd–Ni bimetallic powders, (a) Pd25Ni75, (b) Pd40Ni60, (c) Pd50Ni50 and (d) Pd75Ni25
.
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1
2.26, 9.55 and 9.15 emug–1 for Pd Ni , Pd Ni , Ni nanoparticles, which bring an amphipathic nature to the
25 75 40 60
Pd Ni and Pd Ni , respectively. Basically, the higher Pd–Ni nanoparticles. These amphipathic nanoparticles have
5
0
50
75 25
the proportion of Ni is, the higher the Ms is. By adjusting a tendency to form spherical assemblies because of the for-
the Pd/Ni molar ratio in Pd–Ni bimetals, the magnetic mation of microemulsion droplets in the ethanol/toluene
strength of these ferromagnetic materials could be well con- mixed solution. Thus, the self-assembly behaviours are be-
trolled.
lieved to be affected by the competition between the surfac-
tant interaction and the magnetic dipolar interaction. Be-
cause the magnetic strength is relatively low for Pd-rich
samples, the surfactant interactions play a leading role in
these cases, in which the Pd–Ni nanoparticles assemble to
the spherical beads because of the surfactant interactions,
and then the spherical beads link with each other to form
the beads-on-string structure because of the magnetic di-
polar interaction. For Ni-rich samples, the magnetic dipolar
interactions are strong enough to outweigh the surfactant
interactions to induce the chain-like self-assembly. It can be
concluded that the composition controls the competition
between surfactant interaction and magnetic dipolar inter-
action, and then controls the self-assembly behaviours of
the Pd–Ni nanoparticles.
Figure 5. Magnetic hysteresis loops of (a) Pd25Ni75, (b) Pd40Ni60
,
(c) Pd50Ni50 and (d) Pd75Ni25 at 300 K.
On the basis of the above analyses of XRD patterns,
TEM and SEM images, FTIR spectra and M–H curves, the
mechanism of the composition controlling self-assembly
can be illustrated clearly in Figure 6. The assembly of
chain-like structures resulted from the magnetic properties
and the aggregation of micelles for the surfactant should be
taken into consideration simultaneously during the growth.
In the synthetic system, oleylamine with a long alkyl chain
could be linked to the surface of the Pd–Ni nanoparticles
through a coordination bond, to make the surface hydro-
Figure 6. Schematic illustration of the growth process for the for-
mation of such structures.
In order to study the surface states of such Pd–Ni bimet-
phobic; while the TBAB could be absorbed on the Pd–Ni als, X-ray photoelectron spectroscopy (XPS) was con-
surface by creating an electrostatic double layer, making a ducted. XPS raw spectra of Ni 2p and Pd 3d are shown in
hydrophilic surface.[ As studied by FTIR spectra, the Figure 7. The Ni 2p3/2 XPS peaks at a binding energy (B.E.)
13]
0
oleyl group interdigitate with TBAB on the surface of Pd– of ca. 852 eV can be assigned to Ni , while the peak located
Figure 7. XPS spectra of the Ni 2p (A) and Pd 3d (B) photoelectron peaks in the investigated samples, (a) Pd25Ni75, (b) Pd40Ni60
c) Pd50Ni50 and (d) Pd75Ni25
,
(
.
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at B.E. ca. 856 eV is assigned to Ni2+ or Ni3+ ions,[14] im- Table 1. Comparison of the catalytic activity of different pro-
portions of the Ni–Pd alloy in the hydrogenation of acetophen-
plying that the Ni species were partially oxidized on the
one.[
a]
surface of the samples. The Pd 3d peaks at around 335 eV
5
/2
could be assigned to the zerovalent metallic Pd[ and have Samples Conversion of Selectivity of
15]
Yield of
PE [%]
Yield of
EB [%]
AP [%]
PE [%]
a slight shift from Ni-rich to Pd-rich samples, perhaps be-
cause of an electronic effect.
Pd25Ni75
Pd40Ni60
Pd50Ni50
Pd75Ni25
79
75
93
99
95
90
95
83
76
72
89
83
3
3
4
Recently, Pd–Ni bimetals have shown potential as hydro-
genation catalysts for their excellent performance and rela-
tively low cost compared with pure noble metals.[ Their
composition-dependent surface structure shows great po-
tential in the catalytic reactions. Here, the hydrogenation
catalytic activities of these Pd–Ni assemblies were tested by
the model hydrogenation reaction: the hydrogenation of
acetophenone (AP) to 1-phenylethanol (PE). The reaction
scheme is shown in Scheme 1. The performances of these
Pd–Ni assemblies with various compositions have been
compared by using a batch reactor under the same catalytic
conditions (the mass concentration of catalyst was
16
16]
[
a] Reaction conditions: 10 mL of absolute ethyl alcohol, 0.5 mL of
acetophenone, 10 mg of catalyst. The reaction was carried out for
h under 15 bars of H at 80 °C.
4
2
guidance on the design and development of Pd–Ni bimetal
hydrogenation catalysts. Moreover, compared with pure no-
ble metals, these Pd–Ni bimetal catalysts could easily be
separated from the catalytic reaction solutions for recycling
by using an external magnetic field because of the ferro-
magnetic properties of these Pd–Ni assemblies.
–
1
0
0
.95 gL ; the concentration of acetophenone was
–
1
.41 molL ). The target hydrogenation product, 1-phenyl-
ethanol, is detected along with a small amount of ethyl-
benzene (EB) as a byproduct from the over-hydrogenation
side reaction.
Conclusions
Hierarchical Pd–Ni bimetallic chains have been synthe-
sized by a one-pot synthetic approach in an ethanol/toluene
solution in the presence of oleylamine and TBAB. With the
change of the composition, these Pd–Ni bimetallic assem-
blies show different self-assembly behaviours. For the Ni-
rich sample, the Pd–Ni nanoparticles form simple chain-
like structures, while the Pd–Ni bimetallic chains display a
beads-in-string structure for the Pd-rich samples. The
mechanisms underlying the compositionally controlled self-
assembly have been investigated by XRD, TEM, SEM,
FTIR spectroscopy and VSM analysis. Sufficient evidences
show that the self-assembly of Pd–Ni nanoparticles is gov-
erned by both the surfactant interactions and the magnetic
dipole interactions among the building blocks. The compo-
sition controls the competition of surfactant interactions
and the magnetic dipole interactions, resulting in the diver-
sity of the self-assembly behaviours. The composition also
shows significant influence on the performance of these Pd–
Ni assemblies in the hydrogenation of acetophonene.
Scheme 1. Reaction scheme for the hydrogenation of acetophenone
for the target product PE. (AP: acetophenone, PE: 1-phenyl-
ethanol).
As summarized in Table 1, the conversions of AP are
7
9%, 75%, 93% and 99% for Pd Ni , Pd Ni , Pd Ni
25 75 40 60 50 50
and Pd Ni , respectively. The Pd-rich catalysts show bet-
7
5
25
ter hydrogenation catalytic activity than the Ni-rich cata-
lysts. However, the selectivity of PE decreases and the yield
of EB increases over the Pd-rich catalyst (Pd Ni ) because
7
5
25
of the over-hydrogenation side reaction. When considering
the AP conversion and the selectivity of PE, the Pd Ni
50
50
catalyst shows the highest yield of PE (89%) among these
Pd–Ni bimetal catalysts. The composition-dependent catal-
ysis may be caused by the variation on the active sites on
the surface and electronic effect. XPS results show that Pd
0
in the catalysts consists of Pd in contrast to various chemi-
0
cal states of Ni. The increase of Pd on the surface of the Experimental Section
catalysts provides more active sites for hydrogenation,
Chemicals: Analytical grade ethanol, toluene and oleylamine were
obtained from Beijing Chemical Reagents, China. Ni(Ac) ·4H O,
Acetophenone (C O, 99.5%), cyclohexane and tetrabutyl ammo-
nium bromide (TBAB) were purchased from TianJin GuangFu
The surface areas of the samples are 39.2, 37.6, 67.1 and Fine Chemical Research Institute, China. Palladium(II) acetate [Pd
which leads to an increase in the conversion of AP, as well
as a decrease in selectivity due to the over-hydrogenation
2
2
8
H
8
[
17]
from AP to EB.
2
–1
7
2
2.3 m g for Pd Ni , Pd Ni , Pd Ni and Pd Ni , (Ac) , GR, Ն46 wt.-% Pd] was obtained from Sinopharm Chemical
2
5
75
40 60
50 50
75 25
Reagent Co. Ltd., China. And all the reagents were used without
further purification.
respectively. The catalytic performance of these catalysts
can also be greatly influenced by their surface area. It can
be concluded that the performance of the Pd–Ni bimetallic
2
Synthesis of Composition Controlled Pd–Ni Alloys: Pd(Ac) (0.2 g)
catalysts depends on their compositions, and careful investi- was dissolved in toluene (25 mL) to form a Pd(Ac) solution
2
–
1
gations on the composition effect could give a suggestive (8.0 mgmL ) (a transparent yellow solution A). Then
Eur. J. Inorg. Chem. 2014, 4144–4150
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Ni(Ac)
a Ni(Ac)
tion B).
2
·4H
2
O (0.665 g) was dissolved in ethanol (50 mL) to form
Beijing Higher Education Young Elite Teacher Project (grant
number YETP0484) and the State Key Project of Fundamental
Research for Nanoscience and Nanotechnology (grant number
–1
2
2
·4H O (13.3 mgmL ) solution (a transparent green solu-
2
011CB932402).
We will use Pd25Ni75 as the example. TBAB (0.1 mmol) was placed
in a 50-mL autoclave and solution A (1 mL) and solution B (2 mL)
were injected into the autoclave, and stirred to dissolve the TBAB.
Later oleylamine (10 mL) was added into the autoclave while stir-
ring. The autoclave was then sealed and kept at 200 °C for 24 h in
an oven. The autoclave was cooled down naturally to room tem-
perature after the reaction. The product was washed with the mix-
ture of ethanol and cyclohexane and separated by centrifugation at
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.
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K radiation source. A JTEM-1230 (Japan) transmission electron
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[
[
(
DTGS) detector at room temperature. 32 scans were collected for
–1
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Received: March 12, 2014
[
Published Online: July 23, 2014
Eur. J. Inorg. Chem. 2014, 4144–4150
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim