Synthesis of carbon-supported Pd-Co bimetallic catalysts templated by Co nanoparticles using the galvanic replacement method for selective hydrogenation

Article abstract of DOI:10.1039/c7ra03846a

Pd-Co bimetallic catalysts were prepared by the controlled synthesis of carbon-supported Co catalysts, ranging from single-sites to nanoparticles, via structural transformation of a deposited Co(salen) complex precursor by heat treatment at different temperatures, followed by galvanic replacement with Pd ions. The catalysts were structurally characterized using XRD, TEM, HAADF-STEM, XAFS and XPS. Highly dispersed Pd-rich NPs containing a low concentration of Co species were formed and the original size of the Co NPs was retained. The electronic state of the Pd species was dependent on the size of the bimetallic NPs, and a catalyst with a mean diameter of 10.8 nm was found to be in the most electron-rich state. Not only the particle size, but also the electronic state of Pd play a crucial role in attaining high catalytic activity and selectivity in the selective hydrogenation of phenylacetylene. A Pd-Co catalyst based on a Co catalyst heat-treated at 600 °C showed 92% selectivity at 93% conversion. Furthermore, the Pd-Co catalysts synthesized by the galvanic replacement method showed superior performance to a monometallic Pd catalyst and a Pd-Co alloy catalyst.

Full text of DOI:10.1039/c7ra03846a

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Synthesis of carbon-supported Pd–Co bimetallic
catalysts templated by Co nanoparticles using the
galvanic replacement method for selective
hydrogenation
Cite this: RSC Adv., 2017, 7, 22294
abc
Takeharu Yoshii,a Kazuki Nakatsuka,a Yasutaka Kuwahara,ab Kohsuke Mori
ab
*
and Hiromi Yamashita
Pd–Co bimetallic catalysts were prepared by the controlled synthesis of carbon-supported Co catalysts,
ranging from single-sites to nanoparticles, via structural transformation of a deposited Co(salen) complex
precursor by heat treatment at different temperatures, followed by galvanic replacement with Pd ions.
The catalysts were structurally characterized using XRD, TEM, HAADF-STEM, XAFS and XPS. Highly
dispersed Pd-rich NPs containing a low concentration of Co species were formed and the original size
of the Co NPs was retained. The electronic state of the Pd species was dependent on the size of the
bimetallic NPs, and a catalyst with a mean diameter of 10.8 nm was found to be in the most electron-
rich state. Not only the particle size, but also the electronic state of Pd play a crucial role in attaining
high catalytic activity and selectivity in the selective hydrogenation of phenylacetylene. A Pd–Co catalyst
based on a Co catalyst heat-treated at 600 ꢀC showed 92% selectivity at 93% conversion. Furthermore,
the Pd–Co catalysts synthesized by the galvanic replacement method showed superior performance to
a monometallic Pd catalyst and a Pd–Co alloy catalyst.
Received 4th April 2017
Accepted 18th April 2017
DOI: 10.1039/c7ra03846a
rsc.li/rsc-advances
The galvanic replacement (GR) method has been used over
the last decade for the synthesis of bimetallic nano-
structures.19,20 A pre-formed base metal, such as Co, Ni or Cu,
Introduction
Bimetallic nanoparticles (NPs) have attracted much attention
over the last several decades due to their potential applications
as catalysts.1–6 Ligand and ensemble effects in bimetallic cata-
lysts have been reported to dramatically enhance their catalytic
activity and selectivity.7–9 These effects depend on the compo-
sition, size, morphology and structure of the bimetallic
species,10–12 and therefore control of these properties is impor-
tant in the design of heterogeneous catalysts for specic reac-
tions. Bimetallic nanostructures are generally categorized by
alloy structure, heterostructure, and core–shell structure, and
different nanostructures have been reported to show unique
catalytic performance.13,14 The preparation method is a crucial
factor in determining the nal structure of bimetallic NPs.15–17
Co-reduction of mixed metal ions is the most convenient
method for preparing alloy NPs, whereas sequential deposition
is typically used to prepare core–shell bimetallic NPs. A wide
variety of additional strategies has been developed to synthesize
bimetallic nanostructures.10,18
and transition metal ions with a higher reduction potential,
such as Pd2+, Pt2+ or Au3+, are used as precursors. The base
metal is displaced by the transition metal ions due to the
difference in ionization potential, and the second metal is
redeposited onto the outer surface of the base metal. Various
types of bimetallic nanostructures have been fabricated,
including a core–shell structure, a hollow structure, and
a porous structure, depending on the preparation conditions. A
preparation technique based on GR has been developed for
electro-catalysts, and various bimetallic catalysts show high
electro-catalytic activities for oxygen reduction.21–27 The GR
method has recently also been used to synthesize catalysts for
other reactions, such as hydrogenation reaction,28–33 methane
combustion34 and preferential oxidation reaction.35
In this study, a series of carbon-supported Co species with
different structures, ranging from single-sites to NPs, was used
for the synthesis of Pd–Co by the GR method. Bimetallic
nanoparticle catalysts prepared through the GR process usually
exhibit core–shell21,26,32–34 or hollow/porous20,23,28,35 structures. In
contrast, the Pd–Co NPs fabricated in this study are Pd-rich NPs
with a low Co content that retain the size of the original Co NPs.
Thus, the size of Pd–Co NPs can be controlled by the heat
treatment temperature during preparation of the carbon-
aDivision of Materials and Manufacturing Science, Graduate School of Engineering,
Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan. E-mail:
yamashita@mat.eng.osaka-u.ac.jp; Fax: +81-6-6105-5029; Tel: +81-6-6879-7457
bElements Strategy Initiative for Catalysts Batteries (ESICB), Kyoto University,
Katsura, Kyoto 615-8520, Japan
cJST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
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supported Co species. The electronic state of the Pd species was
varied by changing the size of the Pd–Co nanoparticles, and the
electron-rich Pd species was found to be most active and
selective for the hydrogenation of phenylacetylene. In addition,
the Pd–Co catalysts prepared by the GR method exhibited
performance superior to that of monometallic Pd catalyst and
Pd–Co catalysts prepared by the conventional impregnation
method.
Synthesis of Pd/AC
The reference catalyst of Pd/AC was synthesized by the following
method. Activated carbon (100 mg) was dispersed in 20 mL of
the ethanol/H2O and Pd(NO3)2 aqueous solution (140 mL, Pd
5 mg mLꢁ1) was added dropwise under continuous stirring.
Aer string for 1 h, the solvent was removed under vacuum with
a rotary evaporator. The obtained powder was dried up for 1 day
under vacuum and denoted as Pd/AC.
Experimental
Synthesis of PdCo/AC-600-imp
Materials
The reference catalyst of PdCo/AC-600-imp was prepared using
impregnation method. Co(salen) complex (10 mg) was dissolved
into acetone (20 mL) and the activated carbon (90 mg) was
added to the solution. Aer stirring for 1 h, the solvent was
removed under vacuum with a rotary evaporator. The obtained
powder was dispersed into ethanol/H2O, and Pd(NO3)2 aqueous
solution (140 mL, Pd 5 mg mLꢁ1) was added. Aer string for 1 h,
the solvent was removed under vacuum with the rotary evapo-
Ethanol, methanol, 1,4-dioxane, acetone, styrene(monomer),
ethylbenzene, biphenyl and sodium borohydride [NaBH4] were
purchased from Nakalai Tesque, Inc. (CH3COO)2Co$4H2O and
Pd(II) nitrate solution were obtained from Wako Pure Chemical
Ind. Co., Ltd. N,N0-bis(salicylidene)-ethylenediamine (H2(salen))
and ethynylbenzene phenylacetylene were purchased from
Tokyo Chemical Industry Co., Ltd. Activated carbon (Shirasagi,
M644) was obtained from Osaka Gas Chemicals Co., Ltd.
ꢀ
rator. The obtained powder was heat-treated at 600 C for 2 h
under a 100 mL minꢁ1 nitrogen ow, and will be referred as
PdCo/AC-600-imp.
Synthesis of Co(salen) complex
The Co(salen) complex was prepared by a typical procedure.36
H2(salen) (805 mg) was added into Ar-saturated ethanol (10 mL)
and the solution was heated up to 90 C in an oil bath. (CH3-
COO)2Co$4H2O (375 mg) as a Co precursor was added to the
solution. Aer stirring for 24 h, the solution was cooled, ltered
and washed for removing the remaining H2(salen) and Co2+
ions, followed by drying under vacuum for a day.
Characterization
Powder X-ray diffraction patterns (XRD) were measured using
a Rigaku Ultima IV diffractometer with a semiconductor
analyzer. The Pd K-edge X-ray absorption ne structure analysis
(XAFS) spectra were recorded at the BL-01 at SPring-8, JASRI,
Harima, Japan (Pro. No. 2016B1104 and 2016A1095). In XAFS
measurements, a Si(111) double crystal was used to mono-
chromatize the X-rays from the 2.5 GeV electron storage ring. In
the XAFS measurement, a sample was pretreated with hydrogen
at 200 ꢀC for 1 h, loaded into the in situ cell with plastic windows
and measured in uorescence mode. The EXAFS data was
examined using the EXAFS analysis program, Rigaku EXAFS.
Transmission electron microscopy (TEM) images were obtained
using a eld emission-transmission electron microscope (FE-
TEM; Hitachi Hf-2000) operated at 200 kV. Scanning trans-
mission electron microscopy (STEM) images and elemental
mapping were obtained using JEOL-ARM 200F equipped with
a Kvex energy-dispersive X-ray detector (JED-2300T) operated at
200 kV. X-ray photoelectron spectroscopy (XPS) was performed
with a Shimadzu XPS system, ESCA-3400, using the Mg Ka line
(1253.6 eV). All samples were measured aer hydrogen reduc-
tion at 200 ꢀC for 1 h, and the designated closed container was
utilized for sample setting to prevent reoxidation. Binding
energy was calibrated using the C 1s photoelectronic peak at
284.5 eV.
ꢀ
Synthesis of Co/AC-T
A series of carbon-supported Co catalysts were synthesized
thorough the process we previously reported.37 Co(salen)
complex (50 mg) was dissolved into acetone (100 mL) and the
activated carbon (AC, 450 mg) was added to the solution. Aer
stirring for 1 h, the solvent was removed under vacuum with
a rotary evaporator, and heat-treated at different temperatures
for 2 h under a 100 mL minꢁ1 nitrogen ow. The obtained
powder was denoted as Co/AC-T (Co: 1.8 wt%), in which the T
indicates the heat treatment temperature.
Synthesis of PdCo/AC-T-GR
PdCo/AC-T-GR was prepared using GR method. Co/AC-T (100
mg) was dispersed in 20 mL of ethanol/H2O and an aqueous
solution of NaBH4 (4 mL, 75 mM) was added dropwise under
2+
vigorous stirring at 30 C to reduce Co to Co0. Aer 30 min
ꢀ
stirring, the suspension was centrifuged and washed thor-
oughly with Ar-saturated ethanol/H2O. The product was
dispersed in 20 mL of Ar-saturated ethanol/H2O, and Pd(NO3)2
Catalytic test
aqueous solution (140 mL, Pd 5 mg mLꢁ1) was added dropwise The catalytic performance was evaluated in the selective
ꢀ
under continuous stirring at 30 C for 30 min. Then, the reac- hydrogenation of phenylacetylene. Prior to the catalytic reac-
tion of Pd2+ + Co0 / Pd0 + Co2+ occurred, and Pd NPs were tion, the catalyst were pretreated with hydrogen at 200 C for
ꢀ
formed by replacing Co species. Finally, the suspension was 1 h. Next, the suspension containing methanol (5.0 mL), 1,4-
ltered and dried under vacuum for a day to obtain PdCo/AC-T- dioxane (5.0 mL), phenylacetylene (1.0 mmol), biphenyl (0.5
GR catalysts.
mmol) as an internal standard and the catalyst (10 mg) were
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added into a glass reactor. The reactor was placed in an oil bath
at 30 ꢀC, and the reaction was started by 10 mL minꢁ1 hydrogen
bubbling through the suspension under continuous stirring.
The samples were examined by gas chromatography (Shimadzu,
GC-2014 with TC-1 column).
Results and discussion
Characterization
The crystallinity of samples was investigated by XRD measure-
ments. Fig. 1(A) shows XRD patterns for a series of Co/AC-T
precursors (T ¼ 400, 500, 600 and 800). The broad peak at 40–
50ꢀ for all samples originates from the amorphous structure of
activated carbon. Co/AC-400 exhibited no signicant peaks
associated with Co species. In contrast, Co/AC-500, -600 and
-800 showed clear peaks corresponding to metallic Co at 44.1ꢀ
or to CoO at 42.5ꢀ, and the intensity of the peaks increased with
increasing heat treatment temperature, indicating the forma-
tion and aggregation of Co NPs above 500 ꢀC. XRD patterns for
PdCo/AC-T-GR and PdCo/AC-600-imp are shown in Fig. 1(B). All
samples were measured aer pretreatment with hydrogen at
200 ꢀC for 1 h. PdCo/AC-T-GR (T ¼ 400, 500, 600 and 800)
provided no peaks arising from Co species, unlike their Co/AC-T
counterparts, and no signicant peaks derived from metallic Pd
structure were observed (typically shown at 40.0ꢀ and 46.6ꢀ).38
These results suggest that the crystallinity of Co NPs is reduced
probably due to the atomic-scale replacement with Pd to form
amorphous Pd–Co NPs. In contrast, PdCo/AC-600-imp showed
a peak at around 41.5ꢀ, which is 1.5ꢀ larger than that for the Pd
face centered cubic (fcc) structure and 3.0ꢀ lower than that for
the Co fcc structure. This peak shi indicates the formation of
a Pd–Co bimetallic alloy structure in PdCo/AC-600-imp.39,40
Fig. 2(A) shows transmission electron microscopy (TEM)
images of Co/AC-T. No NPs are observed in Co/AC-400, sug-
gesting the formation of single-site Co species on the support,
as we previously demonstrated.37 In contrast, Co/AC-500 con-
tained distinct NPs with a mean diameter of 5.9 nm, and the Co/
Fig. 2 TEM images of (A) Co/AC-T (T ¼ (a) 400, (b) 500, (c) 600, (d)
800) and (B) PdCo/AC-T-GR (T ¼ (a) 400, (b) 500, (c) 600, (d) 800) and
(e) PdCo/AC-600-imp.
AC-600 and Co/AC-800 samples contained NPs with a mean
diameter of 13.0 nm and 18.8 nm, respectively. These TEM
images, together with the XRD results, indicate that NPs are
produced above 500 ꢀC and that the diameter of the NPs
increases as the heat treatment temperature increases. TEM
images of PdCo/AC-T-GR and PdCo/AC-600-imp subjected to
hydrogen reduction at 200 ꢀC for 1 h are shown in Fig. 2(B).
PdCo/AC-400-GR contained clear NPs with a mean diameter of
3.4 nm, in contrast to Co/AC-400, which suggests the formation
of Pd or Pd–Co NPs by the GR method, followed by hydrogen
reduction. PdCo/AC-500, -600 and -800 also contained distinct
Fig. 1 XRD patterns of (A) Co/AC-T (T ¼ 400, 500, 600, 800) and (B)
PdCo/AC-T-GR (T ¼ 400, 500, 600, 800) and PdCo/AC-600-imp.
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NPs with a mean diameter of 5.9 nm, 10.8 nm and 14.8 nm,
respectively. The size of the NPs in PdCo/AC-500-GR was similar
to that of Co/AC-500, whereas the PdCo/AC-600-GR and -800-GR
NPs were slightly smaller than those of their Co/AC-T counter-
parts. This nding suggests that Co NPs are replaced by Pd
species while maintaining the size of the original Co NPs
throughout the GR process. Thus, the size of the NPs in PdCo/
AC-T-GR is primarily determined by the heat treatment
temperature during Co/AC-T synthesis. PdCo/AC-600-imp as
a reference sample provided NPs with a mean diameter of
14.2 nm.
The crystallinity and composition of the NPs in PdCo/AC-
600-GR were investigated by scanning transmission electron
microscopy (STEM). Fig. 3 shows a high-angle annular dark-
eld (HAADF) STEM image and EDX elemental mapping
images of PdCo/AC-600-GR. Regularly aligned fringes of the Pd
fcc structure are partly visible in the NPs in the HAADF-STEM
image, but fringes are not discernible in most areas of the
NPs, indicating their low degree of crystallization, consistent
with the XRD results. The EDX elemental mapping images
indicate that the NPs are mainly composed of Pd species. Co
species are segregated in the vicinity of the NPs and are also
highly dispersed over the surface of the carbon support.
X-ray absorption ne structure (XAFS) measurements were
performed to investigate the more detailed local structure of the
Pd species. Fig. 4 shows the Pd K-edge X-ray absorption near-
edge structure (XANES) and Fourier transformed extended X-
ray absorption ne structure (FT-EXAFS) spectra of PdCo/AC-
T-GR, PdCo/AC-600-imp, and reference samples. The peak
shapes in the XANES spectra of a series of PdCo/AC-T-GR are
similar to those for Pd foil. The FT-EXAFS spectra of all PdCo/
Fig. 4 Pd K-edge (A) XANES spectra and (B) FT-EXAFS spectra of
PdCo/AC-T-GR, PdCo/AC-600-imp and reference samples.
26
˚
peak attributed to the Pd–Co bond (typically seen at 2.2 A) is
observed in any PdCo/AC-T-GR sample. On the other hand,
˚
˚
PdCo/AC-600-imp shows two strong peaks, at 2.2 A and 2.6 A,
which can be attributed to Pd–Co and Pd–Pd bonds, respec-
tively. This supports the formation of a Pd–Co bimetallic alloy,
consistent with the XRD result.
The above characterization results indicate that Pd2+ ions
displace Co in Co/AC-T during the GR process, followed by the
formation of partially crystallized Pd NPs containing a low
concentration of Co in all samples of PdCo/AC-T-GR. The size of
the original Co NPs does not change signicantly during the GR
process. At the same time, highly dispersed displaced Co
species are deposited on the surface of the carbon support.
Previous use of the GR method for catalyst preparation indi-
cated that the Pd species was replaced on the surface of the Co
NPs with high dispersion, resulting mainly in the formation of
Co@Pd core–shell catalysts.32–34 In contrast, in the present
preparation, Co NPs are almost fully displaced by Pd species in
all samples, and Pd-rich NPs are produced rather than Co@Pd
core–shell NPs, probably due to the small size of the original Co
NPs.
˚
AC-T-GR samples exhibit a strong peak at around 2.6 A, attrib-
uted to Pd–Pd bonds, revealing the presence of Pd NPs. No clear
Catalytic test
The catalytic performance of PdCo/AC-T-GR was examined in
the hydrogenation of phenylacetylene. This is a sequential
reaction in which styrene, produced from phenylacetylene, is
successively hydrogenated into ethylbenzene. Fig. 5 shows the
catalytic performance of PdCo/AC-T-GR (T ¼ 400, 500, 600 and
800), evaluated by the conversion of phenylacetylene and the
selectivity for styrene. As the heat treatment temperature during
ꢀ
Co/AC-T preparation increased from 400 to 600 C, the activity
and selectivity increased, whereas PdCo/AC-800-GR exhibited
lower activity and selectivity than PdCo/AC-600-GR. Activity and
Fig. 3 HAADF-STEM image and EDX mapping of PdCo/AC-600-GR.
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Fig. 6 Pd 3d XPS spectra of PdCo/AC-T-GR, PdCo/AC-600-imp.
is suggested that the Pd–Co ensembles with electron-rich Pd act
as suitable active sites for selective hydrogenation of phenyl-
acetylene. The electron richness of Pd would promote the
charge transfer into the p* orbitals and destabilization of triple
bond of phenylacetylene, which accelerate the hydrogenation of
phenylacetylene to yield styrene.41 On the other hand, the
interaction between electron-rich p-bond of styrene and the
electron-rich Pd species is not as strong as that of the electron-
decient Pd species. Therefore, the electron-rich Pd species
exhibit a lower adsorption energy toward styrene, which prob-
ably suppresses the sequential hydrogenation of styrene to yield
ethylbenzene.31 This is the probable reason for the high activity
and selectivity of PdCo/AC-600-GR possessing electron-rich Pd
species.
Fig. 8 shows the catalytic performance of PdCo/AC-600-GR
and the reference catalysts. Co/AC-600 exhibited no catalytic
activity in this reaction, whereas Pd/AC showed 80% selectivity
and 90% conversion aer 105 min reaction, which is about 12%
lower selectivity than that of PdCo/AC-600-GR. These results
reveal that Pd is active site for the hydrogenation of phenyl-
acetylene in PdCo/AC-600-GR, while selectivity is enhanced by
Fig. 5 (A) Reaction profiles and (B) conversion–selectivity relationship
in the hydrogenation of phenylacetylene over PdCo/AC-T-GR (T ¼
400, 500, 600, 800).
selectivity were altered by the heat treatment temperature of
PdCo/AC-T-GR, and PdCo/AC-600-GR showed the highest
conversion and selectivity among the PdCo/AC-GR-T samples:
92% selectivity and 93% conversion aer 75 min reaction.
In order to examine the reason for the different catalytic
performance in a series of PdCo/AC-T-GR, the electronic prop-
erties of the Pd species in the catalysts were analyzed by XPS
measurements. The Pd 3d spectra of PdCo/AC-T-GR are shown
in Fig. 6. PdCo/AC-400-GR showed a binding energy (BE) peak of
Pd 3d5/2 at 336.0 eV. As the heat treatment temperature
increased, the BE of PdCo/AC-500-GR and PdCo/AC-600-GR
decreased (335.4 and 335.1 eV, respectively), indicating that
the Pd species in PdCo/AC-600 were in a more electron-rich
state compared with the case for PdCo/AC-400. On the other
hand, the BE for PdCo/AC-800 was 335.3 eV, which is higher
than that of PdCo/AC-600. These electronic states of a series of
PdCo/AC-T-GR were probably inuenced by the remaining Co
species incorporated in NPs. Fig. 7 shows the relationship
between the electronic state (represented by the BE peak of the
Pd 3d2/5 spectra of PdCo/AC-T-GR (Fig. 6)) and catalytic activity
and selectivity. A clear correlation between the electronic
properties and catalytic performance was observed, and cata-
lysts with more electron-rich Pd exhibited higher catalytic
Fig. 7 The relationship between the electronic state of Pd and
activity and selectivity in the selective hydrogenation reaction. It conversion of phenylacetylene/selectivity for styrene.
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higher than that of pure Pd catalyst and its activity was higher
than that of a Pd–Co alloy catalyst.
Acknowledgements
The present work was supported by the Grants-in-Aid for
Scientic Research (KAKENHI, No. 26220911 and 26620194)
from the Japan Society for the Promotion of Science (JSPS).
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