Novel AuPd nanostructures for hydrogenation of 1,3-butadiene

Article abstract of DOI:10.1039/c4ta06226d

Exotic AuPd bimetallic nanoflowers (NFs) are facilely synthesized using a microorganism-mediated, cetyltrimethylammonium chloride (CTAC)-directed method at room temperature. The NFs consist of one-dimensional long pedicels and three-dimensional open horns. The effect of cell dosage and feed concentrations of ascorbic acid (AA), CTAC and metal precursors on the morphology of the bimetallic nanostructures were studied. The results showed that all the obtained materials were alloys with Pd-enriched surfaces. The diameters of the horns decreased, while those of the pedicels increased with increasing the feed concentration of Pd precursor. The presence of Pd precursor was vital for the formation of the nanowire part of the NF structure. Furthermore, the AuPd-NF/microorganism materials exhibited excellent catalytic performance and durability toward the hydrogenation of 1,3-butadiene. This journal is

Full text of DOI:10.1039/c4ta06226d

Journal of
Materials Chemistry A
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PAPER
Novel AuPd nanostructures for hydrogenation of
1
,3-butadiene†
Cite this: J. Mater. Chem. A, 2015, 3,
846
4
Huimei Chen, Jiale Huang,*ac Dengpo Huang, Daohua Sun, Minhua Shao*
and Qingbiao Li
ab
a
a
c
abd
Exotic AuPd bimetallic nanoflowers (NFs) are facilely synthesized using a microorganism-mediated,
cetyltrimethylammonium chloride (CTAC)-directed method at room temperature. The NFs consist of
one-dimensional long pedicels and three-dimensional open horns. The effect of cell dosage and feed
concentrations of ascorbic acid (AA), CTAC and metal precursors on the morphology of the bimetallic
nanostructures were studied. The results showed that all the obtained materials were alloys with Pd-
enriched surfaces. The diameters of the horns decreased, while those of the pedicels increased with
increasing the feed concentration of Pd precursor. The presence of Pd precursor was vital for the
formation of the nanowire part of the NF structure. Furthermore, the AuPd-NF/microorganism materials
exhibited excellent catalytic performance and durability toward the hydrogenation of 1,3-butadiene.
Received 16th November 2014
Accepted 8th January 2015
DOI: 10.1039/c4ta06226d
www.rsc.org/MaterialsA
15–17
physical methods for the synthesis of metal nanostructures.
It was thought that metal ions could be adsorbed and reduced
Introduction
Metals have found extensive use in many diverse applications by the microbial surface, resulting in very small nanoclusters
ranging from catalysis to electronics, photonics, information that would gradually grow into metal nanoparticles over the
1
7,18
storage, sensing, imaging, medicine, and photography, as well microorganisms.
However, the shape of the metal nano-
1
as the generation, conversion, and storage of energy. With the structures could not be effectively controlled by microorgan-
development of nanotechnology, the fabrication of metal isms alone. Inspired by the wet chemical synthesis of tunable
19
nanostructures that maximizes the surface area of metals with Au nanorods in the presence of Au seeds; herein, we tried a
tunable shapes and properties has been intensively studied in synthesis of microorganism-bound Au nanoparticles with very
1,2
the past decade. It has been thought that bimetallic nano- small size as seeds for the shape control of one-dimensional
structures might outperform the corresponding monometallic (1D) gold nanostructures at room temperature. Functional Au-
2
nanostructures due to a synergetic effect in catalysis. At nanowire/microorganism nanocomposites could be obtained
2
,3
4
present, various methods, including co-reduction, pyrolysis,
with Pichia pastoris cells (PPCs) and Escherichia coli cells (ECCs)
5
6
seed-mediated method, hydrothermal method, and galvanic in the presence of hexadecyltrimethylammonium bromide
7
–9
20,21
replacement, have been developed for fabricating bimetallic (CTAB).
Interestingly, using cetyltrimethylammonium chlo-
However, shape ride (CTAC) instead of CTAB, this approach could be expanded
control of bimetallic nanostructures based on a versatile to the synthesis of chemically difficult-to-synthesize Au nano-
2,3,6,7,9–14
nanostructures with various shapes.
22,23
strategy still remains challenging.
horns (Au NHs).
Therefore, the microorganism-mediated,
The microbial biosynthesis of metal nanoparticles has been surfactant-directed (MSD) synthesis has been proposed as a new
regarded as a novel and viable alternative to chemical and method for the shape control of metal nanostructures at room
temperature.
a
It is well known that binary metal ions can be simultaneously
adsorbed by microorganisms. The nanoparticles formed by the
reduction of microorganism-bound binary metal ions can act as
Department of Chemical and Biochemical Engineering, College of Chemistry and
Chemical Engineering, Xiamen University, Xiamen 361005, PR China. E-mail: cola@
xmu.edu.cn; kemshao@ust.hk; Fax: +86-592-2184822; Tel: +86-592-2183088
b
Environmental Science Research Center, College of the Environment & Ecology, nuclei for the growth of novel bimetallic nanostructures. The
Xiamen University, Xiamen 361005, PR China
advantage of the MSD approach is that bimetallic nano-
structures can be obtained in one pot at room temperature.
Furthermore, the problem of metal leaching that occurs in
galvanic replacement reactions for synthesizing bimetallic
c
Department of Chemical and Biomolecular Engineering, Hong Kong University of
Science and Technology, Clear Water Bay, Hong Kong, PR China
d
College of Chemistry & Life Science, Quanzhou Normal University, Quanzhou 362000,
PR China
nanostructures can be circumvented. In addition, the bimetallic
†
Electronic supplementary information (ESI) available: The corresponding
histograms of diameter distribution, SEM images, table of XPS peak positions, nanostructures can form application-oriented nano-composites
product conversion and selectivity in 1,3-butadiene hydrogenation, adsorption
with microorganisms. In terms of these factors, herein, we
rates of Au and Pd (1 : 1) adsorbed by the PPCs. See DOI: 10.1039/c4ta06226d
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describe a proof of concept MSD approach to synthesize novel (Rigaku, Japan) and a Quantum 2000 spectrometer using the Al-
AuPd bimetallic nanoowers (NFs) consisting of a one-dimen- Ka line as the excitation source, respectively. The binding
sional pedicel and three-dimensional horn in this study. Such energy was calibrated by C 1s as the reference energy (C 1s ¼
NFs have not been reported hitherto, partially due to the 284.8 eV). Supernatant solutions were decanted aer removing
extreme difficulty in their synthesis by a pure chemical reduc- the composites by centrifuging to analyze the Pd(II) and Au(III)
tion method. The as-obtained AuPd-NF/microorganism nano- concentrations using atomic absorption spectrophotometry
composites can be directly used as an efficient catalyst for the (AAS) (Pgeneral, China). The compositions of Au and Pd in the
hydrogenation of 1,3-butadiene. This work opens a new avenue obtained sample dissolved in aqua regia were also analyzed by
to the shape control of novel bimetallic nanostructures, and AAS.
should nd promising applications in catalysis.
Hydrogenation of 1,3-butadiene
ꢀ
Experimental section
Materials and methods
The catalytic reactions were carried out at 35 C in a 1 cm
diameter xed bed ow stainless reactor under atmospheric
pressure using a feed concentration of 2.13% 1,3-butadiene,
The same Pichia pastoris GS115 cell powder, which was used in
our previous work, was employed in this work as well. Chlor-
oauric acid (HAuCl4), palladium chloride (PdCl2), cetyl-
trimethylammonium chloride (CTAC) and ascorbic acid (AA)
were all purchased from Sinopharm Chemical Reagent Co. Ltd.
in China. Deionized (DI) water was used throughout this work.
25
4
.29% H , and 93.58% N . The temperature was measured
2 2
using a glass tube covered Cr–Al thermocouple located in the
centre of the catalyst bed. The catalyst (15 mg of AuPd/PPC
composites) was diluted with 50 mg of quartz sand before being
loaded into the reactor with a space velocity of 9230 mL h
cat. . The reactor effluent was analyzed online every 20 min
ꢁ1
ꢁ1
g
within a 2 h period using a gas chromatograph (GC) equipped
with an Al O column and a FID detector. The conversion of 1,3-
Preparation of AuPd bimetallic nanostructures
2
3
Three steps were adopted in a typical synthesis of AuPd bime- butadiene and its selectivity to butene (trans-2-butene, 1-
tallic nanostructures. First, dried PPCs (0.005 or 0.01 g) were butene, or cis-2-butene) or butane were calculated.
added to 10 mL of aqueous CTAC solution (1.0, 5.0, 10.0 or 15.0
mM) in a ask. Subsequently, aqueous HAuCl
4
(48.6 mM,
Results and discussion
Synthesis and characterizations of AuPd bimetallic
0
.0514 mL) and H PdCl (0.05 M, 0.050 mL) solutions were
2
4
added to the mixture. Finally, 0.05, 0.1 or 0.5 mL AA (0.1 M) was
added to the mixture. The color of the solution turned from
nanoowers
orange red to light orange immediately aer adding the AA
Effect of metal precursors adding mode. Following our
ꢀ
22,23
solution. The ask was then placed on a shaker (30 C, 150 rpm) previous work on the synthesis of Au NHs,
the MSD
for 1–24 h. A few precipitates were observed at the bottom of the approach was expanded to bimetallic nanostructures in this
asks aer 0.5 h of reaction while the colour of the solution work. Interestingly, as shown in Fig. 1, exotic NFs comprising
turned to black. The nal AuPd/PPC samples were centrifuged one-dimensional pedicels (nanowires) and three-dimensional
at 2000 rpm for 10 min and dispersed in 200 mL of deionized open nanohorns were obtained by reducing HAuCl and
water for further characterizations. It should be noted that the
PdCl (0.25 mM, Au/Pd ratio of 1 : 1, Mode A and B) in the
aqueous Au and Pd solutions were added into the mixture via presence of AA (1.0 mM), PPCs (5 mg) and CTAC (5.0 mM). The
three different modes. For Mode A, Au and Pd precursors were diameters of the pedicels and horns were denoted as d and d

4
H
2
4
,
h
p
simultaneously added into the mixture, and then a 50 min respectively. To the best of our knowledge, similar nano-
interaction between the cells and metal ions was allowed before structures have not been reported hitherto. On the basis of
the addition of AA. For Mode B, Au precursor was added aer Mode A and B, the approximate d
h
of the as-synthesized NFs was
of the NFs
the Pd precursor reacted for 50 min. Mode C was the reverse of ca. 0.26 ꢂ 0.08 mm (Fig. S1 in ESI†). However, the d
p
Mode B.
by Mode A (31.1 ꢂ 6.9 nm) was considerably larger than that by
Mode B (18.8 ꢂ 4.6 nm) (Fig. S1†). As revealed by EDX spectra
Characterizations of AuPd bimetallic nanostructures
Transmission electron microscopy (TEM) characterization was
performed on an electron microscope (Tecnai F30, FEI; Neth-
erlands) with an accelerating voltage of 300 kV. Scanning elec-
tron microscopy (SEM) samples of the suspension were
fabricated by dropping the suspension onto a clean silicon
wafer. SEM observations were carried out on a Hitachi S-4800
SEM system. The sizes of AuPd NFs were measured on the basis
24
of the SEM images. Aer centrifugation, the precipitates were
ꢀ
dried at 50 C in vacuum and used for X-ray diffraction (XRD)
Fig. 1 SEM images of AuPd bimetallic nanostructures with a flower-
like structure synthesized through (a) Mode A and (b) Mode B. The inset
and X-ray photoelectron spectroscopy (XPS) analysis, which
were performed on a Rigaku Ultima IV X-ray Diffractometer in Fig. 1a indicates a model of AuPd flower-like structure.
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Fig. S2†), the local Au/Pd ratio of the NFs was about 1.27. The
Paper
(
overall Pd/Au atomic ratio was further determined to be ca. 0.80
by AAS analysis, lower than the initial ratio of 1.0 (Fig. S3†). The
ꢁ
ꢁ
reduction potential of [AuCl ] /Au ([AuCl ] /Au ¼ +1.002 V vs.
4
4
standard hydrogen electrode (SHE)) was higher than that of
2
ꢁ
2ꢁ
26
ꢁ
[PdCl
4
]
/Pd ([PdCl
4
]
/Pd ¼ +0.591 V vs. SHE). Thus, AuCl
4
2
ꢁ
is easier to be reduced compared to [PdCl ] . As a result, ca.
4
20% Pd was le in the solution, which greatly reduced Pd
leaching compared with galvanic replacement reaction. In
contrast to Mode A and B, Mode C led to the formation of
nanoparticles (NPs) (74.7 ꢂ 8.8 nm, Fig. S4†) rather than NFs.
To prepare the AuPd bimetallic NFs, further characterizations
and synthetic study of the NFs were based on the facile Mode A.
The AuPd NFs prepared on the basis of Mode A were further
characterized by TEM, HRTEM, SAED and EDX line proles.
TEM image (Fig. 2a) indicates that the open nanohorns con-
nected with thin pedicels to form a NF structure. HRTEM
Fig. 3 SEM images of AuPd bimetallic nanostructures synthesized by
Mode A with various CTAC concentrations of (a) 1.0 mM, (b) 10.0 mM
and (c) 15.0 mM. The initial concentrations of the other chemicals were
images (Fig. 2b and c) and the corresponding fast Fourier HAuCl
4
¼ 0.25 mM, H
2
PdCl
4
¼ 0.25 mM and AA ¼ 1.0 mM, while the
dosage of PPCs was 5 mg.
transform (FFT) pattern (insets in Fig. 2b and c) show that the
corresponding blossom and pedicel are polycrystalline with
well-dened lattice planes. The d-spacing of the blossom was
about 0.230 nm, matching the in-between value of the face- nanowire (pedicel) (Fig. 2e) illustrates that they were composed
1
2,27
centered cubic (fcc) Au(111) and Pd(111) planes.
spacing for the adjacent lattice fringes of the pedicel was 0.250 Subsequently, controlled experiments were further performed
Another d of Au and Pd, demonstrating their bimetallic alloy nature.
28
nm, which corresponds to the forbidden 1/3{422}. Moreover, to study the effect of CTAC, AA and Pd concentrations, and cell
the common (111) plane could be determined from the pedicel dosage on the morphology of the AuPd NFs.
(Fig. S5†). The typical SAED pattern (Fig. 2d) from the AuPd NFs
Effect of CTAC. Fig. 3 shows the SEM images of AuPd NFs
shows Bragg reections of {111}, {200}, {220} and {311} of fcc synthesized by varying the feeding concentration of CTAC
AuPd. The EDX elemental line scanning of an individual AuPd (CCTAC), keeping the other conditions the same. By reducing the
Fig. 2 (a) TEM image of AuPd NFs synthesized by Mode A, (b) HRTEM image of a blossom, (c) HRTEM image of a pedicel, (d) SAED pattern of AuPd
NFs, and (e) EDX line profiles of an individual AuPd nanowire (pedicel) corresponding to the framed part (dash) in (a). The insets indicate the
corresponding fast Fourier transform (FFT) pattern.
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CTAC concentration from 5.0 mM (Fig. 1) to 1.0 mM (Fig. 3a),
long nanowires with buds at one end were obtained. The
nanostructures did not change with CTAC concentration
doubling from 5.0 mM to 10.0 mM (Fig. 3b), while the d slightly
h
decreased to 0.25 ꢂ 0.06 mm (Fig. S6a†) from 0.26 ꢂ 0.08 mm.
When CTAC concentration was further increased to 15.0 mM,
the d
h
of the NF structures became much larger (0.51 ꢂ 0.2 mm,
22,23
Fig. 3c and S6b†). As demonstrated in our previous reports,
CTAC had a great impact on the morphology of the nano-
+
structures. The CTA ions released from CTAC could promote
the anisotropic growth of nanostructures. When CTAC was
replaced with octadecyltrimethylammonium chloride (OTAC)
and dodecyltrimethylammonium chloride (DTAC) with
concentration of 5.0 mM, small NPs instead of NF structures
were formed (Fig. S7†). When CTAC was replaced with CTAB,
a
Fig. 5 AuPd bimetallic nanostructures characterized by HAADF-STEM
and STEM elemental mapping (green indicates Au and orange indi-
cates Pd). The nanostructures were synthesized in the presence of AA
with a concentration of (a) 0.5 mM and (b) 5.0 mM. The initial
+
only nanowires were obtained due to the co-existence of CTA
ꢁ
and Br . Thus, CTAC was favorable for the formation of the concentrations of the other chemicals were HAuCl4 ¼ 0.25 mM,
AuPd NFs.
Effect of AA. As a mild reducing reagent, AA also had a strong
2
H PdCl
4
¼ 0.25 mM and CTAC ¼ 5.0 mM, while the dosage of PPCs
was 5 mg.
effect on the morphology of AuPd nanostructures. Fig. 4a and b
depict the SEM images of AuPd nanostructures prepared at the
initial AA concentration (CAA) of 0.5 mM and 5.0 mM, respec-
tively. Interestingly, uniform smooth nanowires without horn-
like structures (Fig. 4a) were obtained by halving the AA
growth of AuPd NPs, which further evolve into AuPd NFs under
the direction of CTAC. The effect of the PPC dosage on the
morphology of the bimetallic nanostructures was further
investigated. Compared with Fig. 1, when the cell dosage was
doubled from 5.0 mg to 10.0 mg, NFs with shorter and thicker
pedicels (Fig. 6b) were attained. Thus, more AuPd NPs were
distributed over the cell surface by increasing the number of
cells. However, AuPd NPs were less concentrated over PPCs,
which prevented the formation of potential AuPd NFs. There-
fore, excess amounts of PPCs could form shorter and thicker
pedicels rather than AuPd NFs. Therefore, the appropriate
amount of PPCs should be determined in order to form well-
dened AuPd NFs. The PPCs have dual functions, i.e. in the
biosorption of binary metal ions and as a platform for prefer-
ential nucleation.
concentration used in Fig. 1. In addition, the d
wires (22.3 ꢂ 4.7 nm, Fig. S8a†) was smaller than that in Fig. 1a
31.1 ꢂ 6.9 nm). By increasing the AA concentration to 5 mM,
p
of the nano-
(
the horn-like structures remained with shorter pedicels. The dh
of the nanohorns in Fig. 4b was 0.29 ꢂ 0.14 mm, which is slightly
larger than that in Fig. 1a (Fig. S8b†). To demonstrate the
distribution of Pd and Au in the nanostructures, STEM
elemental mapping characterization was carried out. Fig. 5a
and b indicate the homogeneous distribution of Au (green) and
Pd (orange) in the NW and NF structures, respectively.
Effect of cell dosage. The effect of the cell dosage (mcell) on
the shape of the nanostructures was also examined. Without
PPCs, irregular petals and rough nanobelts were obtained
Effect of Pd concentration. By comparison with our previous
22,23
work on Au NHs,
AuPd NFs with long pedicel nanostructures
(Fig. 6a). In contrast, AuPd NFs were produced in the presence
were produced by introducing Pd into the system. In other
words, the nanowire morphology (pedicel) was promoted. Thus,
the effect of the initial Pd : Au ratio on the morphology of
nanostructures was investigated. The Pd : Au ratio of X : 5 (X ¼
of PPCs. Our previous studies showed that the biosorption of
17,18
Au(III) and Pd(II) ions by the PPCs was rather rapid.
Thus,
Au(III) and Pd(II) ions can be easily bound to the cells. As a result,
the binding sites act as preferential nucleation sites for the
1
, 3, 5, 7, 9) was adjusted by changing the Pd precursor
Fig. 4 SEM images of AuPd bimetallic nanostructures synthesized by
Mode A at the AA concentration of (a) 0.5 mM and (b) 5.0 mM. The Fig. 6 SEM images of AuPd bimetallic nanostructures synthesized
initial concentrations of the other chemicals were HAuCl
¼ 0.25 mM, through Mode A in the presence of dried PPCs of (a) 0 g and (b) 0.01 g.
PdCl
¼ 0.25
4
H
2
4
¼ 0.25 mM and CTAC ¼ 5.0 mM, while the dosage of PPCs The initial concentrations of the other chemicals were HAuCl
4
was 5 mg.
2
mM, H PdCl
4
¼ 0.25 mM, CTAC ¼ 5.0 mM, AA ¼ 1.0 mM.
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demonstrated that Pd played a crucial role in forming the
9
nanowire morphology in the alloy nanostructure.
To investigate the structures, valence states and elemental
distributions of the prepared materials at different Pd : Au
ratios, XRD, XPS and HAADF-STEM characterizations were
carried out. The XRD results (see Fig. 8a) show that the
diffraction peaks of different Pd : Au ratios are located between
those of pure Au (pdf2 card: 00-002-1095) and Pd (pdf2 card: 01-
087-0635), which can be indexed to the (111), (200) and (220) of
the fcc structure of Au and Pd. The (111) diffraction peak
exhibits a shi from pure Au to pure Pd with increasing the Pd
content (see the inset of Fig. 8b). Also, a good linear relationship
was found between the diffraction angles and the Pd mole
fractions in Fig. 8b. According to Vegard's law, this linear rela-
tionship indicated that the prepared AuPd nanostructures were
alloy and that the composition of each bimetallic nanostructure
Fig. 7 SEM images of AuPd nanostructures synthesized by Mode A at
Pd : Au ratios of X : 5 where X is (a) 1, (b) 3, (c) 7, (d) 9. The initial
11,12,29
was proportional to that of the feeding solution.
concentrations of the other chemicals were HAuCl
4
¼ 0.25 mM, CTAC
¼
5.0 mM and AA ¼ 1.0 mM, while the dosage of PPCs was 5 mg.
XPS was adopted to study the valence states and surface
compositions of Au and Pd at different Pd : Au ratios. Fig. 9a
shows the XPS peaks of Au(0) 4f7/2 and 4f5/2 for the PdAu
nanostructures. The binding energies shi gradually to lower
energies as the Pd content increases. Interestingly, the binding
energies of Pd(0) 3d5/2 and 3d3/2 shi gradually to higher
energies with increasing Pd content, as shown in Fig. 9b. The
binding energies are summarized in Table S1.† The slight shis
in binding energy indicated the changes in the electronic
properties of both Au and Pd on/near the surface of the NF
materials, further verifying the formation of the alloy nano-
concentration while keeping the Au precursor concentration
constant. Fig. 7 shows the SEM images of AuPd nanostructures
synthesized at different Pd : Au ratios, apart from the Pd : Au
ratio of 5 : 5 for Fig. 1a. When a small amount of Pd (Pd : Au
ratio ¼ 1 : 5) was added, petals with rough multi-serrated-like
nanobelts were obtained without pedicels (see Fig. 7a). Well-
dened NFs structures with long thin pedicels were formed with
increasing the Pd : Au ratio to 3 : 5 (see Fig. 7b). The
morphology in Fig. 7b was very similar to that in Fig. 1 (Pd : Au
ratio ¼ 5 : 5). When further increasing the Pd content, pedicel
structures were still retained, while horn-like structures dis-
appeared. Instead, nanoparticles or a two-dimensional broom
at the end of wire appeared (see Fig. 7c and d). It was found that
the diameters of the horns gradually decreased with increasing
27,30
structures.
According to the XPS results, all the AuPd alloy
nanostructures have Pd-enriched surfaces (see the last column
in Table S1†). That is, the Pd : Au ratios of the surfaces were
much higher than the actual ratios derived from the AAS
measurements. Also, the actual Pd/Au ratio was lower than the
theoretical one for each sample (see Fig. S3†). The formation of
the Pd-enriched surface of the AuPd alloy nanostructures may
be due to the different reduction rates of the Au(III) and Pd(II)
species. Considering the reduction potential discussed above, it
can be assumed that Au nuclei formed rst, then followed up by
the Pd concentrations, i.e. d
h
were 0.69 ꢂ 0.27 mm, 0.29 ꢂ 0.09
mm and 0.24 ꢂ 0.06 mm (see Fig. S9a–c†) corresponding to
Fig. 7a–c, respectively. Conversely, the diameters of the nano-
wires increased from 27.8 ꢂ 7.3 nm to 34.8 ꢂ 5.5 nm (see
Fig. S9d–f†). Therefore, the formation of nanowires was attrib-
uted to Pd in this system. In the previous report, Zhu et al. also
11
adding Au and Pd atoms onto the nuclei, leading to alloy
nanostructures with Pd-enriched surfaces being formed.
Fig. 8 (a) XRD patterns and (b) (111) diffraction peak positions of the AuPd bimetallic nanostructures with different initial AuPd molar ratios. The
inset in Fig. 8b shows the XRD patterns in Fig. 8a (rectangle dashed line).
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Fig. 9 XPS patterns of (a) Au 4f and (b) Pd 3d of the AuPd alloy nanostructures with different initial AuPd molar ratios.
Fig. 10 shows STEM and STEM elemental mapping of AuPd difference showed that the Pd ions should occupy some binding
alloy nanostructures at ve Pd : Au ratios, which provide sites over the cell surface, as the Au(III) ions are more easily
insights into both the morphology and distribution of Au and reduced than the Pd(II) ions. When Au precursor was added
Pd with increasing Pd concentration. From le to right, shown prior to the Pd precursor, the Au(III) ions rapidly adsorbed on
in Fig. 10a–e are images of the nanostructures with increasing the cell surface. As a result, only a limited amount of binding
Pd content. From up to down, each colour picture represents sites on the cell surface were available for Pd(II) ions. The
the elemental mappings of one nanostructure. It can clearly be preferential nucleation sites on the cell surface would be
seen that, the elements Au (green) and Pd (orange) are predominantly occupied by Au nuclei, posing a difficulty for
uniformly dispersed in the nanostructures. Furthermore, the adding Pd atoms onto the sites. For Mode B, the Pd(II) ions
morphologies are consistent with the SEM results in Fig. 1a occupied many binding sites, while fewer sites were le for the
and 7.
Au(III) ions, still leading to the same NF structures. Under these
Formation mechanism of AuPd NFs. As discussed above, the circumstances, tandem reactions played an important role in
AuPd NFs were synthesized based on Mode A and Mode B promoting the NF nanostructures. The rst reaction was the
instead of Mode C (i.e. the PPCs interacted with Au(III) ions for reduction of Pd(II) ions to Pd(0) by AA. Then, the resulting Pd(0)
50 min prior to addition of Pd precursor). The pronounced acted as reductants for the reduction of Au(I) (even Au(III)) ions
Fig. 10 From top to bottom, AuPd bimetallic nanostructures characterized by STEM and STEM elemental mapping (green indicates Au and
orange indicates Pd); from left to right, AuPd bimetallic nanostructures synthesized with Pd : Au ratios of X : 5 where X is (a) 1, (b) 3, (c) 5, (d) 7 and
(
e) 9.
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that both NPs and nanowires contained both Au and Pd
elements, and that the percentage Au was higher than that of
+
Pd. The CTA was expected to adsorb onto the crystal nucleus
and thus promote the anisotropic growth of nanostructures.
Aer 15 min, the three-dimensional horn stretched on the cell
surface and the pedicels became longer (see Fig. 11c) owing to
the platforms of the PPCs for large-size nanostructures. It was
noted that some AuPd NPs tended to adhere to the structures,
and these NPs would be consumed via an Ostwald ripening
mechanism later on during the formation of the AuPd NFs.
With further growth, larger-size AuPd NFs were formed (see
Fig. 11d). Eventually, aer ꢃ1 hour, most of the nanostructures
evolved into well-dened AuPd NFs (see Fig. 1a) through
anisotropic growth with the assistance of CTAC and the Ostwald
ripening of small AuPd NPs.
Based on the TEM images, we proposed schematic illustra-
tions for the formation of AuPd NFs, as shown in Fig. 12. First,
the interaction of the PPCs with Au(III) and Pd(II) ions for over 50
minutes led to a balanced adsorption of Au and Pd ions onto the
PPCs (at Stage I), and some Au(III) were reduced to Au(I). Next,
the addition of surplus AA resulted in a further reduction of the
adsorbed Au(I) and Pd(II) to form AuPd nuclei, which acted as
preferential nucleation sites for the formation of AuPd NPs on
PPCs (at Stage II). Then, under the direction of CTAC, the
anisotropic growth of AuPd NWs with bud-like structures at the
Fig. 11 AuPd nanostructures evolution at different times after the
addition of AA: (a) 0 min, (b) 5 min, (c) 15 min and (d) 30 min. The
synthesis condition was the same as in Fig. 1a.
to Au(0) via galvanic replacement. Therefore, we tentatively end of wires was preferred (at Stage III). Finally, well-dened
believe that the tandem reactions were favourable to the addi- AuPd NFs were formed via an Ostwald ripening during the aging
tion of Au atoms onto the nanostructures, even when the of the AuPd NFs (at Stage IV).
nucleation sites were not sufficient for the Au(III) ions, as in the
Catalytic activity of the hydrogenation of 1,3-butadiene. The
case of Mode B.
AuPd-NF/microorganism composites were explored as catalysts
Through careful time-resolved TEM studies, the probable for the selective hydrogenation of 1,3-butadiene. The catalytic
growth mechanism of AuPd NFs on the basis of Mode A was performances of the AuPd-NF/microorganism catalysts with
studied. When HAuCl
PPCs solution, the colour of the solution exhibited an orange composites synthesized by the microorganism-mediated,
red colour at the beginning due to the presence of ligand- CTAB-directed method were also tested for comparison.
4 2
and PdCl were added in the CTAC and different Au/Pd ratios were evaluated. The previous Au-NW/PPC
20
ꢁ
2ꢁ
+
substituted anions such as [AuCl
4
] , [PdCl
4
]
, CTA –Au(III) or Furthermore, Pd-NP/PPC composites by the microorganism-
31–33
[
PdCl (CTA) ] complexes.
Next, Pd(II) and Au(III) ions were mediated, CTAC-directed method (Pd NWs could not be
4 2
both adsorbed on the cell surface. Fiy minutes of adsorption prepared yet) was also evaluated for this reaction. It can be seen
gave rise to a balanced adsorption rate of Pd(II) and Au(III) of ca.
6
6% and 37%, respectively (see Fig. S10†). In the meantime,
17,34
PPCs and CTAC could act as mild reduction agents,
leading
to the colour change from orange red to orange, due to the
reduction of Au(III) to Au(I). The Au(I) anions and Au(I)–CTAC
22
complex were colourless, resulting in the lighter colour of the
reaction solution. Upon the addition of AA, the colour of the
solution changed to light orange immediately. As shown in
Fig. 11a, there are some observable NPs on the cell surface
before the addition of AA. It should be noted that CTAC was a
very weak reductant at room temperature. Therefore, the NPs
largely resulted from the reduction of metal ions with the
17,18
cells.
The nucleation of AuPd NPs grew quickly and nano-
wires (pedicels) appeared on the cell surfaces 5 min aer the
addition of AA. The nanowires preferred to grow at the junction
of the two cells (see Fig. 11b). During this period, the colour of
the solution changed from light pink to dark black, indicating
that most of the Au ions were rstly reduced by PPCs and CTAC,
Fig. 12 Schematic diagram of the formation of AuPd NFs through the
reduction of aqueous HAuCl and PdCl with AA in the presence of
4
2
while most of Pd ions were reduced by AA. EDX data showed PPCs and CTAC.
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Fig. 13 (a) Catalytic performance of the AuPd-NF/microorganism catalysts and (b) the durability of AuPd (1 : 1) NF/microorganism catalysts in
,3-butadiene hydrogenation.
1
from Fig. 13a that the Au-NW/PPC composites were inactive for pedicels increased, while those of the nanohorns decreased
the reaction with very low conversion efficiency. As far as the Pd- with increasing Pd content. In addition, the Pd element
NP/PPC composites were concerned, they exhibited a high contributed to the formation of the nanowire morphology in the
selectivity to butene (trans-2-butene, 1-butene and cis-2-butene). NF structures, and the obtained AuPd/microorganism materials
For the AuPd-NF/microorganism catalysts, with increasing Pd possessed Pd-enriched surfaces. Importantly, the obtained
content, the conversion efficiency increased gradually from AuPd NFs/microorganism materials exhibited excellent catalytic
33.2% to 100%, and the selectivity to butene also improved activity and durability for the hydrogenation of 1,3-butadiene.
signicantly compared with the single Au-NW/PPC and Pd-NP/
PPC catalysts. As reported in the literature for the hydrogena-
35,36
tion of 1,3-butadiene,
the selectivity to butene could be Acknowledgements
enhanced by the addition of a second metal or metal oxide. It is
worth pointing out that, for the AuPd-NF/microorganism cata-
lyst (with a Au/Pd ratio of 5 : 3), the selectivity to total butene
and single 1-butene could reach 96.5% and 50%, respectively,
though the per-pass conversion efficiency was only 65.4%.
However, by further increasing the Pd : Au ratio to 5 : 5, 7 : 5
and 9 : 5, the conversion efficiency could reach 100%, and the
selectivity to butene was slightly lower. The difference in the
selectivity and conversion efficiency can be attributed to the
shi of the Pd 3d binding energy of the NFs at different Pd : Au
ratios (see Fig. 9b). Therefore, the catalytic performance was
composition-dependent. Also, the electron density of Pd
This work was supported by the NSFC projects (no. 21106117
and 21036004). J. H. is grateful to the Fujian Provincial
Administration of Civil Service for the Fujian-Hong Kong joint
postdoctoral fellowship. M.S. was supported by a startup fund
from the Hong Kong University of Science and Technology.
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