Kinetically stabilized Pd@Pt core-shell octahedral nanoparticles with thin Pt layers for enhanced catalytic hydrogenation performance

Article abstract of DOI:10.1021/cs501612g

This study investigates the structural stability of small Pd@Pt core@shell octahedral nanoparticles (NPs) and their shell thickness dependent catalytic performance for p-chloronitrobenzene hydrogenation with H₂. The 6-8 nm Pd@Pt octahedral NPs are prepared by a sequential reduction method, and the characterization results confirm that Pd@Pt octahedral NPs with one to four atomic Pt layers can be controllably synthesized. The Pd@Pt octahedral NPs with one atomic Pt layer demonstrate excellent structural stability with the maintenance of core-shell structures as well as high catalytic stability during cycle to cycle catalytic p-chloronitrobenzene hydrogenation reactions. The alumina-supported Pd@Pt octahedral NPs illustrate a superior catalytic performance relative to individual Pt and Pd and their physical mixtures. Theoretical calculations by density functional theory suggest that the unexpected structural stability for Pd@Pt octahedral NPs with thin Pt shells and their corresponding catalytic stability during hydrogenation reactions can be ascribed to the strong binding between Pt surfaces and reactants/products in catalytic reactions. The enhanced catalytic performance of Pd@Pt octahedral NPs possibly originates from the core-shell interaction, which adjusts the electronic state of surface Pt atoms to be suitable for selective p-chloronitrobenzene hydrogenation.

Full text of DOI:10.1021/cs501612g

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Article
Kinetically Stabilized Pd@Pt Core-shell Octahedral Nanoparticles with
Thin Pt Layers for Enhanced Catalytic Hydrogenation Performance
Peipei Zhang, Yibo Hu, Baihai Li, Qiuju Zhang, Chen Zhou, Hongbo
Yu, Xuejun Zhang, Liang Chen, Bryan W. Eichhorn, and Shenghu Zhou
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs501612g • Publication Date (Web): 16 Jan 2015
Downloaded from http://pubs.acs.org on January 20, 2015
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Kinetically Stabilized Pd@Pt Core-shell Octahedral
Nanoparticles with Thin Pt Layers for Enhanced Catalytic
Hydrogenation Performance
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Peipei Zhang,†, ┴, # Yibo Hu,†, # Baihai Li,§ Qiuju Zhang,† Chen Zhou,† Hongbo Yu,†
Xuejun Zhang,┴ Liang Chen,†, * Bryan Eichhorn,‡, * and Shenghu Zhou†, *
†Ningbo Institute of Materials Technology and Engineering, Chinese Academy of
Sciences, 1219 Zhongguan West Road, Ningbo, Zhejiang 315201, P. R. China.
‡Department of Chemistry and Biochemistry, University of Maryland, College Park,
Maryland 20742, United States
§School of Energy Science and Engineering, University of Electronic Science and
Technology of China, Chengdu, Sichuan 611731, P. R. China
┴College of Applied Chemistry, Shenyang University of Chemical Technology,
Shenyang, Liaoning 110142, P. R. China
Author Contributions
#P. Zhang and Y. Hu are equal contributors
*Corresponding author
Fax: (+86) 574-86685043
E-mail: zhoush@nimte.ac.cn; eichhorn@umd.edu; chenliang@nimte.ac.cn
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Abstract
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This study investigates the structural stability of small Pd@Pt core@shell
octahedral nanoparticles (NPs) and their shell thickness dependent catalytic
performance for p-chloronitrobenzene hydrogenation with H2. The 6~8 nm Pd@Pt
octahedral NPs are prepared by a sequential reduction method, and the
characterization results confirm that Pd@Pt octahedral NPs with 1~4 atomic Pt layers
can be controllably synthesized. The Pd@Pt octahedral NPs with one atomic Pt layer
demonstrate excellent structural stability with the maintenance of core@shell
structures as well as high catalytic stability during cycle-to-cycle catalytic
p-chloronitrobenzene hydrogenation reactions. The alumina supported Pd@Pt
octahedral NPs illustrate a superior catalytic performance relative to individual Pt, Pd
and their physical mixtures. Theoretical calculations by density function theory
suggest that the unexpected structural stability for Pd@Pt octahedral NPs with thin Pt
shells and corresponding catalytic stability during hydrogenation reactions are
ascribed to the strong binding between Pt surfaces and reactants/products in catalytic
reactions. The enhanced catalytic performance of Pd@Pt octahedral NPs possibly
originates from the core-shell interaction, which adjusts the electronic state of surface
Pt atoms to be suitable for selective p-chloronitrobenzene hydrogenation.
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Keywords: Pd@Pt, core-shell, nanocatalysis, hydrogenation, p-chloronitrobenzene
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1. Introduction
Bimetallic nanoparticles (NPs) have been applied in many research fields, such
as catalysis,1-6
electrochemistry7-15 and biological sciences.16-19 Among the
structures of bimetallic NPs, core@shell NPs have attracted intense interest due to
saving precious metals by positioning relatively cheap metals in the cores20 and the
performance enhancement by the core-shell interaction.19 Various core@shell
structures have been successfully synthesized, such as Pd@Pt,13,21-24 Rh@Pt25 and
Ni@Pt.26,27 Ideal core@shell structures are those with just one atomic layer of the
shell metal because the thinnest shell can obtain the highest shell metal dispersion and
the strongest core-shell interaction, thus resulting in the highest specific activity.19
Although the literature has reported successful synthesis and application of core-shell
structures, the structural stabilities of core@shell NPs are rarely studied,22,28
especially for core-shell structures with thin shells.
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The structural stabilities of core@shell NPs are determined by the miscibility of
core and shell metals, the shell thickness, and surface-adsorbate interaction during
reaction conditions. Pd@Pt bimetallic NPs were intensely studied due to their
enhanced performance in heterogeneous catalysis,23 hydrogen storage,21 and oxygen
reduction reactions.13,29 However, theoretical analysis and experimental studies
indicated the poor structural stability of Pd@Pt core-shell structure. The difference of
cell parameters between Pd and Pt is less than 1.3 %, resulting in a PdPt alloy
structure at equilibrium. Moreover, because the surface free energy of Pd is lower
than that of Pt, the structures with Pd enriched surfaces are more stable than Pd@Pt
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core-shell structure.30-32 Furthermore, in the hydrogen-related applications, the
hydrogen adsorbate will draw Pd atoms to the topmost surface because of the stronger
hydrogen binding on Pd relative to Pt.33,34 Therefore, a pure Pt shell on a Pd@Pt NP is
difficult to maintain during catalytic hydrogenation reactions, especially for Pd@Pt
with 1~2 atomic Pt layers.
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Accordingly, the synthesis of thermodynamically unstable Pd@Pt core-shell
structure is difficult. The Toshima group obtained the inverse Pt@Pd structure by
co-reduction of PdCl2 and H2PtCl6 by ethanol. The formation of Pt@Pd core-shell
structure is enhanced by the favorable coordination between Pd and the protecting
agent- polyvinylpyrrolidone (PVP).35 Literature has reported some special techniques
for the synthesis of Pd@Pt core-shell structures. Toshima and co-workers reported a
sequential reduction method for synthesis of Pd@Pt core-shell structure using
sacrificial hydrogen.23 Xia and co-workers reported the synthesis of relatively large
Pd@Pt nanocubes using a slow injection of Pt precursors to Pd nanocube colloid at a
sufficiently high temperature.22
The transformation of Pd@Pt NPs to other more stable structures such as PdPt
alloys or Pt@Pd structures has been observed at mild conditions. For example,
Crooks and co-workers employed Pd@Cu NPs as precursors in an attempted
synthesis of Pd@Pt NPs with one atomic Pt layer (Pd@Pt1L) by galvanic
displacement. Surprisingly, they obtained Pt@Pd reverse core-shell structures through
the inversion of Pd@Pt structures during the synthetic process at room temperatures.36
The Kitagawa group discovered the transformation of Pd@Pt2L NPs to PdPt alloys
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simply by a process of hydrogen absorption/desorption at 100 °C.28 The above
findings confirm the poor structural stability of Pd@Pt core-shell structures with thin
Pt shells, consistent with the theoretical calculation.
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Here, we reported kinetically stabilized small Pd@Pt core-shell NPs with
enhanced catalytic performance for p-chloronitrobenzene (p-CNB) hydrogenation
with H2. The 6~8 nm Pd@Pt octahedral NPs with 1~4 atomic Pt layers are
synthesized by a simple glycol sequential reduction method. The alumina supported
Pd@Pt1L NPs display a superior catalytic performance relative to individual Pd,
individual Pt and their physical mixtures. Characterization studies confirm that
Pd@Pt core@shell structures are maintained after multiple cycles of catalytic
hydrogenation. Moreover, the surface PdPt alloy NPs obtained by thermal treatment
of Pd@Pt1L at high temperatures are effectively converted to Pd@Pt core-shell
structures after catalytic hydrogenation of p-CNB. These findings show that
Pt-adsorbate interaction not only stabilizes the Pd@Pt structure, but can also induce
segregation of Pt to the surface from a PdPt alloy structure during catalytic reactions.
Theoretical calculations by density function theory (DFT) suggest that the superior
structural stability of Pd@Pt1L is due to the strong bonding of reactants and products
to the Pt surfaces during catalytic reactions. The resultant catalytic performance
enhancement possibly originates from the core-shell interaction. The schematic
demonstration of kinetically stabilized Pd@Pt NPs during catalytic p-CNB
hydrogenation is illustrated in Scheme 1.
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NH2
Cl
NO2 + H2
Cl
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Cycle-to-cycle
Pt precursor
Reducing
Structure maintenance
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Pd NPs
Pd@Pt1L
Pd@Pt1L
Thermal
treatment
Ostwald
ripening
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Pd atom
Pt atom
Structure transformation
surface PdPt alloy
Pd@Pt
NH2
Cl
NO2
Cl
+ H2
Scheme 1. Synthesis of Pd@Pt core-shell NPs and their stabilization during p-CNB
hydrogenation reactions.
2. Experimental Section
2.1. Chemicals
Palladium chloride (PdCl2) and ammonium hexachloroplatinate (IV)
((NH4)2PtCl6) were purchased from Aladdin. Polyvinylpyrrolidone (PVP-K30, GR),
ethylene glycol (EG, AR), absolute ethyl alcohol (AR), acetone (AR) and
p-chloronitrobenzene (p-CNB, CP) were purchased from Shanghai Chemical Reagent
Company. Aluminum oxide powders were purchased from Qingdao Haiyang
Chemical Co., Ltd.. All of the reagents were used as received without further
purification.
2.2. Syntheses of Individual Pd, Individual Pt, Pd@Pt Core-shell NPs, and
Alumina Supported Nanocatalysts
Synthesis of Individual Pd NPs. In a typical synthesis, 0.352 mmol of PdCl2,
120.0 mg of PVP-K30, and 25.0 ml of EG were transferred into a 50 ml three-necked
round-bottomed flask, and the resultant mixture was heated and maintained at 60 °C
for 30 minutes with magnetic stirring under an N2 atmosphere. The mixture was then
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heated and maintained at 90 °C for 30 minutes, and then the dark solution was further
heated and maintained at 150 °C for 2 hours. The resultant solution was cooled down
to room temperature, followed by centrifugation with acetone. The obtained black
powdery product was then re-dispersed in ethyl alcohol for future use.
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Synthesis of Individual Pt NPs. The synthesis of Pt NPs was similar to that of
Pd NPs. In a typical synthesis, 0.352 mmol of (NH4)2PtCl6, 120.0 mg of PVP-K30,
and 25.0 ml of EG were transferred into a 50 ml three-necked round-bottomed flask,
and the resultant mixture was heated and maintained at 90 °C for 30 minutes with
magnetic stirring under an N2 atmosphere. The mixture was then heated and
maintained at 130 °C for 30 minutes. The solution was further heated and maintained
at 150 °C for 2 hours. The Pt NP collection process is similar to that of Pd NPs.
Synthesis of Pd@Pt Core-shell NPs. Various Pd@Pt NPs with different Pd/Pt
ratios were synthesized by a sequential reduction method. For the synthesis of Pd@Pt
NPs with a 1/1 ratio of Pd/Pt, 10.0 g of the above mentioned as-synthesized Pd colloid
containing 0.126 mmol of Pd, 10.0 ml of EG, 0.126 mmol of (NH4)2PtCl6, and 30.0
mg of PVP-K30 were mixed in a 50 ml three-necked round-bottomed flask under
magnetic stirring under an N2 atmosphere. The resultant mixture was heated and
maintained at 90 °C for 30 minutes. The solution was then heated and maintained at
130 °C for 30 minutes to reduce the Pt precursors, and then was heated and
maintained at 150 °C for 2 hours. The Pd@Pt nanoparticle collection process is
similar to that of Pd NPs. The Pd@Pt core-shell NPs with Pd/Pt ratios of 1/0.75, 1/0.5,
1/0.25, and 1/0.125 were synthesized by a similar method through adjusting the
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amount of Pt precursors, and other conditions were same as those of Pd@Pt NPs at
the Pd/Pt ratio of 1/1. The Pd@Pt NPs with Pd/Pt ratios of 1/1, 1/0.75, 1/0.5, 1/0.25,
and 1/0.125 were denoted as Pd@Pt-1/1, Pd@Pt-1/0.75, Pd@Pt-1/0.5, Pd@Pt-1/0.25,
and Pd@Pt-1/0.125, respectively.
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Synthesis of Al2O3 Supported Nanocatalysts. A defined amount of the above
mentioned as-synthesized colloid was centrifuged with acetone once, and the obtained
black powdery product was re-dispersed in 30.0 ml ethanol. The resultant colloid was
transferred into a 50 ml three-necked round-bottomed flask, and 1.0 g of Al2O3
calcined at 550 °C for 2 hours (specific surface area-185 m2/g) was then added into
the colloid. The resultant mixture was heated with magnetic stirring at 60 °C and
purged with N2 to remove the ethanol. The resultant gray powdery solid was further
heated at 60 °C in an oven overnight. The product was then washed with acetone once
and ethanol four times to thoroughly remove surfactants, followed by drying in an
oven at 60 °C overnight to obtain Al2O3 supported nanocatalysts.
2.3. Characterization
Transmission electron microscopy (TEM) images were obtained by using a
JEOL 2100 transmission electron microscope operated at 200 kV. The high angle
annular dark field scanning transition electron microscopy (HAADF-STEM) images,
phase mapping and line scans by energy-dispersive spectroscopy (EDS) were
obtained using a TitanG2 80-200 ChemiSTEM equipped with a Super-X EDX
detector system Cs-corrected STEM. X-ray photoelectron spectra (XPS) were taken
on AXIS ULTRA DLD multifunctional X-ray photoelectron spectroscope with an Al
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source. The data processing was performed using CasaXPS software. The contents of
Pt and Pd in the supernatant solution after centrifugation were determined with a PE
Optima 2100DV inductive coupled plasma optical emission spectrometer (ICP-OES).
The diffuse reflectance flourier transform infrared spectra (DRIFT-IR) with CO
probes were obtained using a Nicolet-6700 Fourier transform infrared spectrometer.
The alumina supported nanocatalysts were pretreated with H2 at 100 °C for 30
minutes at a gas flow rate of 20 ml/min, and subsequently purged with Ar at 100 °C
for 20 minutes. The samples were then cooled down to 30 °C, and recorded as the
DFIRT-IR background spectrum. The alumina supported nanocatalysts were further
treated with pure CO (99.99%) at a gas flow rate of 20 ml/min at 30 °C for 30 minutes,
and subsequently purged with Ar to remove the free CO before the DRIFT-IR spectra
were recorded.
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2.4. Catalytic p-CNB hydrogenation with H2
Hydrogenation of p-CNB with H2 at Ambient Pressure. A 100 ml three-necked
round-bottomed flask was filled with 0.100 g of alumina supported nanocatalysts,
50.0 ml of absolute ethanol and 3.0 g of p-CNB. The mixture was heated and
maintained at 45 °C with vigorous stirring. The reaction system was kept in an H2
atmosphere with a hydrogen flow for defined reaction time. The liquid products were
analyzed by a gas chromatography equipped with a flame ionization detector. For
cycle-to-cycle catalytic reactions, the catalysts were collected after catalytic reactions
by centrifugation with ethyl alcohol and drying in an oven at 60 °C overnight.
Because some of the supported catalysts were lost during the recovery process, in the
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subsequent cycle reactions, proportionally p-CNB and absolute ethanol were used to
keep the ratio of p-CNB/catalysts and p-CNB/solvent same as those of the first
catalytic run.
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Hydrogenation of p-CNB with H2 at the H2 partial pressure of 4.0 MPa.
Typically, a 500 ml high-pressure agitated autoclave was filled with 0.200 g of
alumina supported nanocatalysts, 200 ml of absolute ethanol and 12.0 g of p-CNB.
The autoclave was then purged with hydrogen at least three times at room temperature.
The hydrogenation reactions were carried out at 200 °C and hydrogen partial pressure
of 4.0 MPa with a stirring rate of 500 rpm for 3 hours.
3. Results and Discussion
3.1. Synthesis and Characterization of Pd@Pt Core-shell NPs
Pd@Pt octahedral NPs were synthesized by a simple sequential reduction
method. Pd NPs were first prepared by reduction of Pd precursors by glycol using
PVP as capping agents. Pt was further deposited on the Pd seeds to form core@shell
structures. The ICP-OES study suggests that greater than 99.5 % of the Pd and Pt in
starting materials are reduced to form NPs.
TEM images of Pd@Pt are shown in Figure 1, and their corresponding size
analyses are shown in Figure 2. Pd NPs in Figure 1a show well-faceted shapes with an
average particle size of 5.6 nm while Pt NPs in Figure 1b are relatively spherical with
an average particle size of 4.2 nm. Pd@Pt NPs with the Pt/Pd ratios of 0.125/1, 0.25/1
and 0.5/1 are illustrated in Figure 1c, 1d and 1e, respectively, and octahedral shapes
are observed for those NPs. The octahedral shapes with sharp corners are evidenced
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by the inserts of Figure 1c, 1d and 1e, and are consistent with the literature.37 In
contrast, at higher Pt/Pd ratios of 0.75/1 in Figure 1f and 1/1 in Figure 1g, the Pd@Pt
NPs contain octahedral shapes with some other shapes, indicating that the influence of
Pd seeds on the shape of Pt shells decreases at higher Pt/Pd ratios. Size analyses of
Pd@Pt NPs in Figure 2c-2g show a continuous increase in particle size from 5.9 to
7.2 nm when the Pt/Pd ratios increase from 0.125/1 to 1/1, indicating the continuous
Pt deposition on Pd nanoparticle surfaces.
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c)
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20 nm
5 nm
5 nm
5 nm
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5 nm
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20 nm
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20 nm
Figure 1. TEM images showing a), Pd; b), Pt; c), Pd@Pt-1/0.125; d), Pd@Pt-1/0.25;
e), Pd@Pt-1/0.5; f), Pd@Pt-1/0.75; g), Pd@Pt-1/1; h), Pd@Pt-1/0.25/Al2O3 after 5
cycles of catalytic p-CNB hydrogenation reactions at 45 °C and ambient H2 pressure;
i), Pd@Pt-1/0.25/Al2O3 after catalytic p-CNB hydrogenation reactions at 200 °C and
4.0 MPa of H2. The inserts show enlarged TEM images of individual nanoparticles.
Scale bars in a)-i) are 20 nm; scale bar in the inserts are 5nm.
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c)
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Particle Size (nm)
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Figure 2. Size analyses of individual and core-shell NPs showing a), Pd; b), Pt; c),
Pd@Pt-1/0.125; d), Pd@Pt-1/0.25; e), Pd@Pt-1/0.5; f), Pd@Pt-1/0.75; g),
Pd@Pt-1/1ꢀh), Pd@Pt-1/0.25 after 5 cycles of catalytic p-CNB hydrogenation
reactions at 45 °C and ambient H2 pressure; i), Pd@Pt-1/0.25/Al2O3 after catalytic
p-CNB hydrogenation reactions at 200 °C and 4.0 MPa of H2.
XPS spectra of Pd@Pt with different Pt/Pd ratios are illustrated in Figure S1. The
XPS curve fitting employs the method of Doniach and Sunjic,38 and the binding
energy assignment is according to the literature.36,39,40 Table 1 summarizes the atomic
ratios of Pt/Pd for Pd@Pt NPs by XPS. Approximately 3.5~6.1 % atomic percent of
Pdδ+ and Pt2+ species are observed for Pd@Pt NPs presumably due to the colloid
exposure to air. The Pt/Pd ratio for the Pd@Pt-1/1 NPs by EDS is 1.02/1.00, which is
close to the 1/1 Pt/Pd ratio in the starting materials. However, the Pt/Pd ratio for the
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Pd@Pt-1/1 NPs obtained by XPS is 3.13/1.00, which is significantly higher than that
obtained by EDS. The same phenomenon is also observed for the Pd@Pt-1/0.5 and
Pd@Pt-1/0.25 NPs, which shows two times higher Pt/Pd ratios by XPS than those by
EDS. Since XPS is a surface sensitive technique, these data reveal compositions of
the outer layers for core@shell NPs. Therefore, the higher Pt/Pd ratios by XPS are
consistent with the core@shell structure with Pd cores and Pt shells.
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Table 1. Atomic Ratios of Pt/Pd with Different Oxidation States from the XPS
Analysis for Pd@Pt Core-shell NPs.
Samples
Pd0/Pdδ+
Pt0/Pt2+
Pt/Pd (XPS)
3.13/1.00
Pt/Pd (EDS)
1.02/1.00
Pd@Pt-1/1
96.4 / 3.6
96.5 / 3.5
Pd@Pt-1/0.5
Pd@Pt-1/0.25
95.0 / 5.0
93.9 / 6.1
96.0 / 4.0
94.0 / 6.0
1.15/1.00
0.52/1.00
0.52/1.00
0.26/1.00
The relative atomic concentration of Pd and Pt by line-scans and elemental phase
mapping by EDS for Pd@Pt NPs are shown in Figure 3. The HAADF-STEM images
and relative atomic concentration of Pd and Pt by the EDS line-scans for the
Pd@Pt-1/0.25 are shown in Figure 3a and 3b, respectively. The highest Pd
concentration is found at the center of Pd@Pt particles while the highest Pt
concentration is found at the peripheries of particles, suggesting a core@shell
structure with Pd cores and Pt shells. The Pd cores and very thin Pt shells are clearly
observed in the combined Pd and Pt phase mapping in Figure 3f. The HAADF-STEM
images and relative atomic concentration of Pd and Pt by the EDS line-scans for the
Pd@Pt-1/1 are shown in Figure 3g and 3h, respectively. The highest Pt atomic
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concentration at the peripheries of NPs is more clearly observed in Figure 3h than that
of Pd@Pt-1/0.25 in Figure 3b due to the thicker Pt shells at a higher Pt/Pd ratio of 1/1
in the Pd@Pt-1/1 NPs. The thicker Pt shells are also confirmed by the combined
phase mapping of Pd and Pt for the Pd@Pt-1/1 NPs in Figure 3l.
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a)
b)
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Pt-L
Pd-L
800
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0.0
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Position (nm)
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h1)500
g)
Pt-L
Pd-L
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k)
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Position (nm)
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20 nm
Figure 3. a)-f) EDS line-scans and phase mapping for Pd@Pt-1/0.25 NPs: a),
HAADF-STEM image, and the particle with yellow line is for EDS line-scan; b),
relative atomic concentration of Pd and Pt by EDS line-scans; c), selected area for
EDS phase mapping; d), Pd phase mapping; e), Pt phase mapping; f), the combined
Pd and Pt phase mapping. g)-l) HAADF-STEM image, relative atomic concentration
of Pd and Pt by EDS line-scans, selected area for EDS phase mapping, Pd phase
mapping, Pt phase mapping, the combined Pd and Pt phase mapping for Pd@Pt-1/1,
respectively. Scale bars in a) and g) are 20 nm; scale bars in c)-f) and i)-l) are 10 nm.
DRIFT-IR spectra with CO probes of alumina supported Pd, Pt and Pd@Pt NPs
with different Pt/Pd ratios are shown in Figure 4. As shown in Figure 4a, Pd/Al2O3
exhibits a band at 1946 cm-1, which is a characteristic band of bridged CO on Pd
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sites.41-44 The band near 2100 cm -1 for linear CO on Pd sites is not observed for
Pd/Al2O3, which is consistent with the reported spectra of well-faceted Pd NPs.44
Figure 4b shows the DRIFT-IR spectrum of alumina supported Pt NPs. Two
characteristic bands at 2065 cm-1 and 2088 cm-1 are clearly observed, and the former
is ascribed to linear CO on Pt step and kink sites and the latter can be attributed to
linear CO on Pt terrace sites.23,24,45 A 1860 cm-1 band for bridged CO on Pt sites is not
observed for these particles,24 indicating a low concentration of bridged CO.46,47 The
DRIFT-IR spectrum of alumina supported physical mixtures of Pd and Pt NPs is
shown in Figure 4c. The bands at 2087 and 2063 cm-1 for linear CO on Pt sites and
1947 cm-1 for bridged CO on Pd sites are observed, consistent with the spectrum of
the physical mixture of Pd and Pt NPs.
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1946
A=0.05
a)
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2088
b
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)
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2087
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d
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1869
e
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2058
f
)
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Wavenunber (cm-1)
Figure 4. DRIFT-IR spectra with CO probes showing a), Pd/Al2O3; b), Pt/Al2O3; c),
the physical mixture of Pd/Al2O3 and Pt/Al2O3 with a Pt/Pd ratio of 1/1; d),
Pd@Pt-1/0.125/Al2O3; e), Pd@Pt-1/0.25/Al2O3; f), Pd@Pt-1/1/Al2O3.
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DRIFT-IR spectra of Pd@Pt/Al2O3 with different Pt/Pd ratios are shown in
Figure 4d-4f. The alumina supported Pd@Pt-1/0.125 nanocatalyst shown in Figure 4d
exhibits bands at 2036 and 1908 cm-1, where the former is assigned to linear CO on Pt
sites and the latter is ascribed to bridged CO on Pd sites. The co-presence of CO
binding on Pt and Pd sites is consistent with an incomplete Pt coverage on the Pd
seeds at a low Pt/Pd ratio of 0.125/1. When the Pt/Pd ratio is increased to 0.25/1
(Figure 4e), a slight hump at 1869 cm-1 for bridged CO on Pt sites, bands at 2040 and
2072 cm-1 for linear CO on Pt sites are present while the bands around 1950 cm-1 for
bridged CO on Pd sites are absent, indicating a complete Pt coverage on the Pd seeds
at the Pt/Pd ratio of 0.25/1. Similarly, the spectra of supported Pd@Pt-1/1 NPs in
Figure 4f only shows CO absorption on Pt surface (2087 and 2058 cm-1), further
confirming a core@shell structure.
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The individual Pt NPs exhibit two characteristic bands at 2065 and 2088 cm-1. In
contrast, the Pd@Pt-1/0.25 NPs with a full Pt coverage illustrate two linear CO
absorption bands at 2040 and 2072 cm-1 with one band at 1869 cm-1 for bridged CO.
When the Pt/Pd ratio increases to 1/1, the Pd@Pt-1/1 NPs show CO absorption bands
at 2058 and 2087 cm-1, which is very similar to those of individual Pt NPs, indicating
that the influence of Pd cores decreases with the thickness of Pt shells increasing. For
the Pd@Pt-1/0.125 NPs with an incomplete Pt coverage, the bands for the linear CO
on Pt sites merge into one band at 2036 cm-1 for linear CO on Pt step and kink sites.
The presence of only linear CO binding on Pt step and kink sites is consistent with the
presence of mostly isolated Pt atoms at a low Pt/Pd ratio of 0.125/1.
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According to the diameter of Pt atom and close-packed model of Pt on Pd seeds,
one complete Pt atomic layer accounts for 0.45 nm.36 From these structural models,
the particle size of a Pd@Pt NP with one complete atomic Pt layer (Pd@Pt1L) using
5.6 nm Pd seeds should be 6.1 nm while the particle sizes of Pd@Pt2L and Pd@Pt3L
are 6.5 and 7.0 nm, respectively. Actually, the measured particle size of the
Pd@Pt-1/0.25, Pd@Pt-1/0.5 and Pd@Pt-1/1 NPs are 6.1 nm, 6.5 nm and 7.2 nm,
respectively, which is consistent with the Pd@Pt1L, Pd@Pt2L, and Pd@Pt3~4L
assignments, respectively. It can be concluded that Pd@Pt NPs have a core-shell
structure with Pd cores and Pt shells, and the thickness of Pt shell can be well
controlled by the current method. At the Pt/Pd ratio of 0.25/1, the Pd@Pt-1/0.25 is in
line with the Pd@Pt1L structure.
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Xia and co-workers has reported a growth model from Pd cubes to Pd octahedra
through the Pd cuboctahedron intermediate.37 In this study, the Pd seeds are
cuboctahedrons in Figure 1a, and the Pd@Pt core-shell NPs are octahedra in Figure
1c, 1d and 1e, which is well consistent with a growth model from Pd cuboctahedrons
to Pd@Pt octahedra. The schematic demonstration of the formation of Pd@Pt
core-shell octahedral NPs is shown in Scheme 2.
Pt atom
Pd atom
Pd@Pt 1/0.125
Pd
Pd@Pt 1/0.25
Scheme 2. The schematic demonstration of the formation of Pd@Pt core-shell
octahedral NPs with one Pt atomic layer from Pd cuboctahedral NP seeds.
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3.2. Catalytic p-CNB Hydrogenation over Supported Pd@Pt NPs
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Catalytic p-CNB hydrogenation with H2 was used to investigate the catalytic
performance of alumina supported Pd@Pt NPs. As for p-CNB hydrogenation, the
major product is p-chloroaniline (p-CAN) with aniline (AN) and 4-aminophenol
(4-AP) as by-products from de-chlorination reaction.48-51
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The catalytic performances of p-CNB hydrogenation over supported
nanocatalysts at ambient H2 pressure and 45 °C are shown in Table 2. The Pt/Al2O3
catalysts illustrate 79.5 % of p-CNB conversion and 81.5 % of p-CAN selectivity
while Pd/Al2O3 catalysts showing 61.7 % of conversion and 45.8 % of p-CAN
selectivity. For the Pd@Pt-1/0.125/Al2O3 catalysts, p-CNB conversion and p-CAN
selectivity are between those of supported Pd and Pt nanocatalysts. In contrast, the
Pd@Pt-1/0.25/Al2O3 catalysts illustrate 98.5 % of conversion and 82.1 % of p-CAN
selectivity, showing a better performance than Pd/Al2O3, Pt/Al2O3, their physical
mixture (Pd+Pt)-1/0.25/Al2O3. With continuously increasing Pt/Pd ratios, the
Pd@Pt-1/0.5/Al2O3, Pd@Pt-1/0.75/Al2O3 and Pd@Pt-1/1/Al2O3 show decreased
conversions with enhanced p-CAN selectivity. The Pd@Pt-1/0.25/Al2O3 was also
selected for hydrogenation at critical conditions, and show enhanced p-CNB
conversion and p-CAN selectivity at higher temperatures and higher H2 pressure (See
Table S1). The effects of reaction temperatures and reaction time on the catalytic
performance over Pd@Pt-1/0.25/Al2O3 are shown in supporting information (Figure
S2 and S3). Basically, with reaction temperatures and reaction time increasing, the
p-CNB conversion and p-CAN selectivity are increased, and at the reaction time of
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3.0 hours and reaction temperature of 45 °C, the p-CNB conversion and p-CAN
selectivity over Pd@Pt-1/0.25/Al2O3 reach 98.5 % and 82.1 %, respectively.
Table 2. The Catalytic Performance of p-CNB Hydrogenation with H2 over
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Supported Individual and Core@shell Nanocatalysts.
Selectivity (%)
Catalysts
Conversion (%)
p-CAN
81.5
AN
4-AP
else
3.7
Pt/Al2O3
79.5
61.7
74.3
70.7
98.5
97.4
91.2
88.0
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(Pd+Pt)-1/0.25/Al2O3
Pd@Pt-1/0.125/Al2O3
Pd@Pt-1/0.25/Al2O3
Pd@Pt-1/0.5/Al2O3
Pd@Pt-1/0.75/Al2O3
Pd@Pt-1/1/Al2O3
11.4
11.0
Reaction conditions: p-CNB-2.0 g; supported catalysts-0.100 g; EtOH-50 mL; 0.10 MPa H2;
reaction temperature-45 °C; reaction time-3.0 hours; speed of agitation-500 rpm. Pd/Al2O3 (Pd
loading-0.54 wt%); Pt/Al2O3 (Pt loading-1.00 wt%); physical mixture and core@shell catalysts
(fixed Pd loading-0.54 wt%, Pt loading changed with the molar ratio of Pt/Pd).
The Pd@Pt/Al2O3 nanocatalysts illustrate a Pt shell thickness dependent catalytic
performance for p-CNB hydrogenation. The Pd@Pt-1/0.125/Al2O3 catalysts with an
incomplete Pt shell show a mediate catalytic performance between Pd and Pt
nanocatalysts, which is consistent with the structure of an incomplete Pt shell and
some naked Pd surfaces. The Pd@Pt-1/0.25/Al2O3 catalysts (Pd@Pt1L/Al2O3)
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illustrate a significantly enhanced p-CNB conversion with slightly enhanced p-CAN
selectivity, showing the advantage of core@shell structure for catalytic reactions. The
Pd@Pt-1/0.5/Al2O3 catalysts (Pd@Pt2L/Al2O3) demonstrate the best catalytic
performance with the 97.4% of conversion and 87.9% of p-CAN selectivity,
indicating the most suitable core-shell interaction obtained at the Pt/Pd ratio of 0.5/1
for catalytic p-CNB hydrogenation. When the thickness of Pt shell of Pd@Pt NPs
continuously increases, the conversions over Pd@Pt-1/0.75/Al2O3 and
Pd@Pt-1/1/Al2O3 catalysts decrease, which suggests the decreased core-shell
interaction at higher Pt/Pd ratios.
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3.3. Structural and Catalytic Stability of Pd@Pt NPs with One Atomic Pt Layer
The Pd@Pt1L/Al2O3 nanocatalysts (Pd@Pt-1/0.25/Al2O3) were selected to study
the catalytic stability of Pd@Pt nanocatalysts. Table 3 illustrates the results of
cycle-to-cycle p-CNB hydrogenation reactions over Pd@Pt1L/Al2O3. From cycle 1 to
cycle 5, the p-CNB conversion over Pd@Pt-1/0.25/Al2O3 is highly stable while
p-CAN selectivity increases from 82.5 % in cycle 1 to 89.3 % in cycle 2, and is stable
in the following cycles. In this study, the amounts of p-CNB and solvent decrease
according to the weight of the used catalysts recovered from each cycle, and no fresh
catalysts are added into the system. Due to the loss in the catalyst recovery process, 5
cycles of catalytic p-CNB hydrogenation reactions were carried out. This study
strongly suggests that the Pd@Pt1L is catalytically stable for cycle-to-cycle p-CNB
hydrogenation with H2.
Table 3. Cycle-to-cycle p-CNB Hydrogenation Reaction over Pd@Pt1L/Al2O3.
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aCycle
index
p-CNB
(g)
Conversion
(%)
p-CAN
Catalyst (g)
selectivity (%)
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0.100
0.077
0.069
0.055
0.047
2.00
1.55
1.38
1.10
0.93
98.4
99.6
98.5
98.7
99.2
82.5
89.3
89.5
87.2
90.6
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aReaction conditions: Pd@Pt-1/0.25/Al2O3; EtOH in cycle 1-50.0 mL, EtOH of the following
cycle decreased according to the weight of p-CNB at the fixed ratio of EtOH/p-CNB; 0.10 MPa
of H2; reaction time-3.0 hours; reaction temperature-45 °C; agitation speed-500 rpm.
DRIFT-IR study with CO probes was carried out to study the structural stability
of Pd@Pt1L NPs after catalytic reactions. Figure 5 illustrates DRIFT-IR spectra of the
Pd@Pt-1/0.25/Al2O3 nanocatalysts after catalytic reactions. As shown in Figure 5b,
the DRIRT-IR spectrum of Pd@Pt-1/0.25/Al2O3 after 5 cycles of hydrogenation
reactions at 45 °C and ambient H2 pressure only shows the linear CO band on Pt sites
at 2053 cm-1 and bridged CO band on Pt sites at 1877 cm-1, confirming the
maintenance of the core@shell structure during cycle-to-cycle reactions. Compared
with the spectrum of as-prepared Pd@Pt-1/0.25/Al2O3 in Figure 4e, the linear CO
band at 2072 cm-1 disappears, suggesting the occurrence of surface reconstruction
during catalytic reactions.
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A=0.05
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2063
2087
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2053
2037
2043
1877
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d)
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-1
cm )
Wavenunber
(
Figure 5. DRIFT-IR spectra with CO probes showing a), the physical mixture of
Pd/Al2O3 and Pt/Al2O3 with a 1/1 Pt/Pd ratio; b), Pd@Pt-1/0.25/Al2O3 after 5 cycles
of p-CNB hydrogenation at ambient H2 pressure and 45 °C; c), Pd@Pt-1/0.25/Al2O3
after p-CNB hydrogenation at 4.0 MPa H2 and 200 °C; d), Pd@Pt-1/0.25/Al2O3 after
p-CNB hydrogenation at 4.0 MPa H2 and 250 °C.
The Pd@Pt1L/Al2O3 catalysts also demonstrate the structural stability during
catalytic p-CNB hydrogenation at critical reaction conditions. As shown in Figure 5c,
the Pd@Pt-1/0.25/Al2O3 catalysts after catalytic reactions at 200 °C and 4.0 MPa of
H2 only exhibit a linear CO band on Pt sites at 2037 cm-1 with the absence of the band
for CO binding on Pd sites, confirming the maintenance of core@shell structures
during catalytic hydrogenation at critical reaction conditions. For catalytic
hydrogenation at an even higher reaction temperature of 250 °C and 4.0 MPa of H2,
although by-product formation is more severe at 250 °C than that at 200 °C (Table S1),
the Pd@Pt-1/0.25 nanocatalysts still maintain a core@shell structure as evidenced by
the presence of only linear CO band on Pt sites at 2043 cm-1 in Figure 5d.
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The EDS line-scans and phase mapping of the used Pd@Pt1L/Al2O3 catalysts are
shown in Figure 6. As illustrated in Figure 6a-6f, the core-shell structure with Pd
cores and Pt shells is clearly observed for the Pd@Pt-1/0.25/Al2O3 after 5 cycles of
catalytic p-CNB hydrogenation at 45 °C and ambient H2 pressure, which is consistent
with the DRIFT-IR result in Figure 5b. Moreover, the TEM images of
Pd@Pt-1/0.25/Al2O3 after cycle-to-cycle reactions in Figure 1h and size analysis in
Figure 2h suggest that Pd@Pt-1/0.25 maintained the same octahedral shapes and a
nearly same average size of Pd@Pt NPs as those of as prepared Pd@Pt-1/0.25 NPs,
further confirming the structural stability of Pd@Pt1L NPs after cycle-to-cycle
reactions. The phase mapping of supported Pd@Pt-1/0.25 after catalytic reaction at
200 °C and 4.0 MPa of H2 is shown in supporting information (Figure S4). Due to the
Ostwald ripening process during high temperature catalytic reaction, the particle size
increases (Figure 1i and 2i), but the phase mapping still clearly demonstrates that the
Pt atoms are mainly on the surfaces of NPs, which is consistent with the DRIFT-IR
result in Figure 5c. The maintenance of a pure Pt shell in the presence of Ostwald
ripening at high temperature catalytic reactions suggests that some factors during
catalytic reaction help to keep the Pt surface, and will be discussed later.
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b)
100
Pt-L
Pd-L
a)
c)
d)
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60
10 nm
10 nm
10 nm
e)
f)
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20
10 nm
5 nm
5 nm
0.0
5 Position (nm) 10
Figure 6. a)-f) showing EDS line-scan and phase mapping for Pd@Pt-1/0.25/Al2O3
after 5 cycles of p-CNB hydrogenation: a) HAADF-STEM image of selected particles
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for line-scan; b), relative concentration of Pd and Pt by EDS line-scan; c), selected
area for phase mapping; d), Pd phase mapping; e), Pt phase mapping; f), the combined
Pd and Pt phase mapping. Scale bar in a) is 5 nm; scale bars in c)-f) are 10 nm.
The thermal stability of Pd@Pt1L/Al2O3 in the Ar and H2 atmosphere was also
studied, and their corresponding DRFIT-IR spectra are shown in Figure S5 in
Supporting Information. The Pd@Pt1L/Al2O3 after H2 treatment at 4.0 MPa and 250
oC exhibits the CO absorption bands for Pt sites as well as Pd sites, suggesting the
transformation of Pd@Pt core-shell structures to surface PdPt alloy structure.
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Moreover, at an even higher temperature of 550 C, the Ar or H2 treatment can fully
transform Pd@Pt core-shell structure into a structure with a pure Pd surface as
evidenced by the only presence of CO absorption bands for Pd sites.
3.4. Discussion in the Content of Structural Stability and Core-shell Interaction
of Pd@Pt/Al2O3 NPs
According to the theoretical analysis and experimental data,28,36,52 the Pd@Pt
NPs with thin Pt layer should only exist at low temperatures and low hydrogen
pressures. However, in this study, the Pd@Pt1L NPs are quite stable during catalytic
hydrogenation reactions. It is speculated that the p-CNB reactant and p-CAN product
in catalytic hydrogenation bond to Pt more strongly relative to Pd, and the stronger
bonding stabilizes the Pt topmost surface.
Theoretical calculations by DFT theory was carried out to prove the stronger
binding between Pt surfaces and reactants/products (see supporting information,
Figure S6 and Table S2). The binding energy of p-CNB on Pt (111) surface is -4.56
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eV while that on Pd (111) surface is -3.28 eV. Moreover, the binding energies of
p-CAN on Pt (111) surface and Pd (111) surface are -4.80 and -4.40 eV, respectively.
Apparently, p-CNB and p-CAN bond to Pt (111) surface more strongly than they bond
to Pd (111) surfaces. The stronger bonding of p-CNB and p-CAN to Pt surfaces by
theoretical calculations strongly supports that the energy favorable bindings of
reactants and products on Pt sites keep the Pt atoms on the surface layer for Pd@Pt
NPs during catalytic reactions.
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The experimental data also support that the reactants and products in catalytic
p-CNB hydrogenation could stabilize the Pt surfaces of Pd@Pt. As shown in Figure
S5e and S5f, the surface PdPt alloy structures obtained by thermal treatment of
Pd@Pt1L/Al2O3 transforms into Pd@Pt core-shell structures by further catalytic
o
p-chloronitrobenzene hydrogenation at 100 C and 1.0 MPa of H2. The above
experiments strongly suggest that the reactants and products during catalytic p-CNB
hydrogenation not only stabilize the Pt surface of Pd@Pt core-shell NPs, but can also
induce segregation of Pt atoms to the surfaces from PdPt alloy structures at relative
high temperatures and H2 pressures.
The catalytic performance of the Pd@Pt1L core-shell structure is superior to Pd,
Pt, and the physical mixture of Pd and Pt, and is highly stable during cycle-to-cycle
hydrogenation reactions. The superior catalytic stability obviously originates from the
superior structural stability. In this study, the amount of surface Pt atoms increases
with the Pt/Pd ratios increasing since the Pd loadings are fixed in all core-shell
catalysts. It can be concluded that the Pd@Pt1L/Al2O3 illustrate the highest specific
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activity among core-shell catalysts with full Pt coverage. Considering the nearly same
amount of surface Pt atoms of Pd@Pt1L (Pd@Pt-1/0.25, surface Pt loading-0.25 wt%)
and individual Pt (4.2 nm, surface Pt loading-0.26 wt%) according to the theoretical
model,53 the enhanced catalytic performance for catalytic p-CNB hydrogenation is
ascribed to the core-shell interaction, which adjusts the electronic state of the surface
Pt atoms of Pd@Pt core-shell NPs to be suitable for selective p-chloronitrobenzene
hydrogenation. At the same time, the hydrogen adsorption is not decreased due to the
favorable hydrogen adsorption on the Pd subsurface layer since hydrogen adsorption
on Pd is a more energy favorable process than hydrogen adsorption on Pt,21 thus
resulting in an enhanced catalytic performance.
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4. Conclusions
The Pd@Pt core-shell structures are synthesized by a simple sequential glycol
reduction method. The characterization reveals that the Pd@Pt NPs with 1~4 Pt
atomic layers can be controllably synthesized by adjusting the Pt/Pd ratios in the
sequential reduction method. The Pd@Pt1L NPs are obtained at the Pt/Pd ratio of
0.25/1 when 5.6 nm Pd NPs are used as seeds for deposition of Pt. The catalytic
performances of p-CNB hydrogenation with H2 over Pd@Pt core-shell NPs are
dependent on the thickness of Pt shells. At the Pt/Pd ratio of 0.25/1 and higher, the
Pd@Pt core-shell NPs show better catalytic performance in p-CNB hydrogenation
than Pd, Pt, and the physical mixtures of Pd and Pt. The Pd@Pt1L NPs demonstrate
superior structural stability either during cycle-to-cycle p-CNB hydrogenation
reactions at mild conditions or during the catalytic reactions under high temperatures
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and high H2 pressures. The unexpected superior structural stability and corresponding
catalytic stability of Pd@Pt NPs with one atomic Pt layer are ascribed to the strong
binding between Pt surfaces and reactants/products during catalytic reactions, where
the reactant and product molecules stabilize the Pt surfaces by strong coordination.
The superior catalytic performance over Pd@Pt NPs is possibly due to the core-shell
interaction. The findings in this study is helpful to design kinetically stablized
core@shell bimetallic NPs with thin shells, and could be extended to other catalytic
reactions.
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Acknowledgments
S. Zhou thanks the Ministry of Science and Technology of China (Grant No.
2012DFA40550) for financial supports. H. Yu thanks the financial support from
Ningbo Municipal Natural Science Foundation (Grant No. 2013A610038).
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Table of contents
+ H2
Cl
NO2
NH2
Cl
Pt
Pd NPs Pd@Pt1L
Thermal Ostwald
treatment ripening
Pd@Pt1L
Pd
Pt
surface PdPt alloy
Pd@Pt
NH2
Cl
NO2
Cl
+ H2
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