Fabrication, characterization and electrochemical properties of porous hollow palladium spheres

Article abstract of DOI:10.1016/j.jallcom.2015.01.065

In the present study, porous hollow palladium particles with uniform size distribution have been successfully prepared by templating method with silica sphere as template materials. Experimental results show that hydrazine hydrate is a preferable reducing regent to prepare pure palladium layer by electroless plating rather than sodium hypophosphite. Silica sphere templates can be easily removed after eroded in alkali solutions, and then palladium particle with hollow structure and a specific surface area of ～10.26 m²/g can be prepared. Furthermore, electrochemical properties of the porous hollow palladium were also investigated and the results reveal that it has superior ethanol oxidation ability.

Full text of DOI:10.1016/j.jallcom.2015.01.065

Accepted Manuscript
Fabrication, characterization and electrochemical properties of porous hollow
palladium spheres
Qingquan Kong, Ying Liu, Lixian Lian, Jing Zhang, Li Wang
PII:
S0925-8388(15)00120-6
DOI:
http://dx.doi.org/10.1016/j.jallcom.2015.01.065
JALCOM 33107
Reference:
Journal of Alloys and Compounds
To appear in:
Received Date:
Revised Date:
Accepted Date:
13 November 2014
9 January 2015
10 January 2015
Please cite this article as: Q. Kong, Y. Liu, L. Lian, J. Zhang, L. Wang, Fabrication, characterization and
electrochemical properties of porous hollow palladium spheres, Journal of Alloys and Compounds (2015), doi:
http://dx.doi.org/10.1016/j.jallcom.2015.01.065
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Fabrication, characterization and electrochemical properties of
porous hollow palladium spheres
Qingquan Kong a, Ying Liu a, b, , Lixian Lian a, Jing Zhang a, Li Wang a
a School of Materials Science and Engineering, Sichuan University, Chengdu 610065,
PR China
b Key Laboratory of Advanced Special Material & Technology, Ministry of Education,
Chengdu 610065, PR China
Abstract: In the present study, porous hollow palladium particles with uniform size
distribution have been successfully prepared by templating method with silica sphere
as template materials. Experimental results show that hydrazine hydrate is a
preferable reducing regent to prepare pure palladium layer by electroless plating
rather than sodium hypophosphite. Silica sphere templates can be easily removed after
eroded in alkali solutions, and then palladium particle with hollow structure and a
specific surface area of ~10.26m2/g can be prepared. Furthermore, electrochemical
properties of the porous hollow palladium were also investigated and the results
reveal that it have superior ethanol oxidation ability.
Keywords:
Hollow;
Porous
materials;
Microstructure;
Homogeneous;
Electrochemical properties
 Corresponding: Tel.: +86 28 85405332; fax: +86 28 85405332.
E-mail address: liuying5536@163.com (Y. Liu)
1

1. Introduction
Recently, attentions have been paid to palladium materials for their great use in
the research of metal-hydrogen isotope system, catalysis, hydrogen sensor, etc [1-4].
Spongy palladium powders were commonly utilized as active materials in the
hydrogen isotope displacement process [4, 5]. However, the low surface area
(~0.438m2/g) and nonuniform microstructure will reduce hydrogen isotope exchange
probability. Palladium materials were also utilized as catalyst for high-performance
oxygen reduction reaction (ORR). In some researches, catalytic performance of Pd
particles with hollow structure or porous structure can be dramatically increased due
to their large surface area and unique morphology [6-8]. Accordingly, the fabrication
of uniform and spherical hollow Pd particles with controllable size and desired
morphology is noteworthy, which is a key material for the functional development of
devices.
Templating methods have been developed to fabricate hollow spheres by using
template particles such as polystyrene beads, emulsion-templated silica beads or silica
colloids [8-10]. In the former researches, carbon and other impurities were the major
causes of deactivation and premature degradation of palladium catalysts [11, 12].
Thus, carbon type materials should be avoid when choosing a proper template. Cho et
al reported that well-defined hollow Pd nanoshell catalysts had been prepared by
employing Co nanoparticles as sacrificial templates [13]. Due to the relatively lower
oxidation potential of Co, the dissolution of Co metal is accompanied by the reduction
of the Pd metal precursor that results in the replacement of Co with Pd via the
2

galvanic replacement reaction (GRR). Co impurities in the Pd bulk also may lead to a
locally different interaction potential of Pd and H [1], which is the main reason for the
solubility decrease in the β-phase. Compared with the other template materials,
silica colloids possess better monodispersed, wider size range, and homogeneous [14],
which can be used as template to prepare porous hollow Pd materials. Most
importantly, silica colloids can be easily removed.
In this article, we report the fabrication of homogeneous porous hollow Pd
sphere by templating method with silica colloids sphere as a template material. Effect
of reduction agent and plating temperature on the resultant materials were studied.
Moreover, electrochemical properties of these resultant porous hollow Pd materials
towards ethanol oxidation in alkali condition were also investigated.
2. Experimental
2.1 Preparation of silica colloids sphere
Silica colloids sphere were prepared by a modified stöber method. In briefly, 6 ml
of TEOS (Kelong Ltd., China) were rapidly added into a mixture solution of 80mL
ethanol (Kelong Ltd., China), 40mL deionized water, 20mL ammonium aqueous
solution (28%, Kelong Ltd., China) and 10mol/m3 KCl under vigorous agitation. Then
the mixture was stirred at room temperature for 24 h. The resulting silica particles
were centrifugally separated from the suspension, washed with deionized water and
ethanol, and dried.
2.2 Preparation of Pd@SiO2 particles
Silica colloids spheres were sensitized and activated in acidic SnCl2 and PdCl2
3

solutions firstly, and then electroless plating of Pd were take place in a mixture
solution of 0.01M PdCl2, 40g/L ethylene diamine tetraacetic acid (EDTA, Kelong Ltd.,
China) and 450mL/L ammonia aqueous solution under agitation for 30min. The bath
temperatures were 50℃, 60℃ and 70℃ (abbreviation to Pd@SiO2I-1, Pd@SiO2I-2,
Pd@SiO2I-3, respectively). Hydrazine hydrate diluted with distilled water (50vt%)
was added dropwise, the drop volume and interval are about 0.1 ml and 10 s. As a
comparison, the electroless plating of Pd were taken place in a mixture solution of
4ml/L HCl(38%), 27g/L NH4Cl, 160ml/L ammonium aqueous solution (28%) and
10g/L sodium hypophosphite (abbreviation to Pd@SiO2II). The bath temperature was
60℃. After electroless plating, the resultant product were centrifuged and washed with
deionized water.
2.3 Preparation of porous hollow Pd particles
The prepared Pd@SiO2 particles were corroded in 1M NaOH aqueous solution at
60℃ to remove silica colloids. The resultant materials were centrifuged and washed
with deionized water, and then dried in a vacuum chamber. Afterward, porous hollow
Pd particles can be obtained.
2.4 Characterization
The samples were characterized with a field-emission scanning electron microscope
(FESEM, XL30SFEG) equipped with an X-ray energy spectrometer (EDS) and
transmission electron microscope (TEM) for microstructure and elemental analyses.
X-ray diffraction patterns were recorded by an X-ray diffractometer (XRD, DX-2000)
with Cu Kα radiation. Brunauer - Emmett - Teller (BET) surface areas of porous Pd
4

samples were measured by a N2 adsorption apparatus. Electrochemical measurements
were performed in a standard three-electrode cell using a CHI660E electrochemical
workstation in a three-electrode electrochemical cell (500mL) with a Pt plate
electrode as a counter electrode, a saturated calomel electrode (SCE) as a reference
electrode. The work electrode was coated by the as-prepared porous hollow palladium
powder. In detail, 0.4mg porous hollow palladium powder placed on a freshly
polished glassy carbon (GC) electrode with a diameter of 4mm with Nafion solution.
Then, cyclic voltammograms were obtained in 1.0 mol dm−3 KOH + 0.8M
CH3CH2OH at different scan rate. Moreover, ethanol electro-oxidation performance
were characterized in a deaerated mixture solution of 1.0 mol dm−3 KOH with
different concentration of ethanol. All of these electrochemical experiments were
carried out at ambient temperature (~25 ℃). Current density was calculated referred to
the geometric electrode area of GC electrode.
3. Results and discussion
Fig. 1 FE-SEM images and XRD pattern of silica colloids spheres.
Fig. 1 shows the FE-SEM images and XRD patterns of silica colloids spheres.
Homogeneous silica colloids sphere with particle size of ~500nm can be prepared.
The XRD pattern shown in Fig. 1b reveals that the silica colloids spheres prepared by
present study are amorphous.
Fig. 2 FE-SEM images and XRD patterns of Pd@SiO2 particles after electroless
5

plating by different reductions (a) Pd@SiO2I, (b) Pd@SiO2II, (c) XRD pattern of (a)
and (b), (d) EDS pattern of Pd@SiO2I, (e) EDS pattern of Pd@SiO2II, respectively.
Fig. 2 shows FE-SEM images and XRD patterns of Pd@SiO2 particles after
electroless plating by different reductions. As shown in Fig. 2a, the surface of the
silica colloids after plating with hydrazine hydrate as reduction agent was coated
uniformly and dense with Pd nanoparticels. In contrast, Pd shell of Pd@SiO2II
particles after plating with sodium hypophosphite as reduction agent was loosely (Fig.
2b). The distinct microstructures of Pd@SiO2 particles were caused by different
plating chemistry of hydrazine-based bath and hypophosphite-based bath [15]. Fig. 2c
shows the XRD patterns of as-prepared Pd@SiO2 particles. From the image, only Pd
peak can be detected in Pd@SiO2I, which reveals that almost all silica colloids
spheres were coated with Pd. XRD pattern of Pd@SiO2I particles is also sharper than
Pd@SiO2II. Moreover, Pd15P2 peak can be detected in Pd@SiO2II, which reveals that
P impurity can be maintained in Pd shell after plating, which is consistent with EDS
results of Pd@SiO2II shown in Fig. 2e. Phosphorus in Pd-P solid solutions or its
dissolution products will block the ethanol adsorption and led to a decrease in the
electrocatalytic activity in pure Pd materials [16]. In contrast, only Si, O and Pd can
be detected in Pd@SiO2I (Fig. 2d). Palladium was deposited on the substrates with
hydrazine hydrate as reduction agent in the electroless-plating bath according to the
following reactions:
2Pd2 4e  2Pd0
(1)
6

N2H4  4OH  N2  4H2O 4e
2Pd2  N2H4  4OH  2Pd0  N2  4H2O 4e
(2)
(3)
Only N2 can be generated in the reactions. Palladium nitride is thermodynamically
unstable at low pressure and will decompose into metallic Pd and solid N2 at ambient
pressure [17]. As a result, nitrogen can’t remain in the Pd and pure Pd layer can be
obtained. Thus, hydrazine hydrate is a preferable reducing regent to prepare pure
palladium layer by electroless plating rather than sodium hypophosphite.
Fig. 3 FE-SEM images of Pd@SiO2Pd@SiO2I particles after electroless plating at
different temperature. (a) 50℃, (b) 60℃, (c) 70℃.
Plating temperature should also be taken into account for it has large effect on the
morphology of the resultant materials. FE-SEM images of Pd@SiO2I particles after
electroless plating at different temperature are shown in Fig. 3. As shown in Fig. 3a,
Pd shell was unsound when plating at 50℃. Pd nanoparticles were completely
grown, forming a integrated Pd shell when the plating temperature was 60℃, which is
beneficial to maintain the integrity after removed of template materials. When the
plating temperature was increased to 70℃, more independent Pd nanoparticles can be
produced besides Pd@SiO2 particles. Also, some nanoparticles were mutual fused.
Moreover, the particle sizes of Pd increase with the increase of plating temperature.
Thus, in order to obtain robust porous hollow Pd after removal of template materials,
plating bath temperature of ~60℃ should be selected.
7

Fig. 4 Characterization of porous hollow palladium particles
Hydrogen fluride (HF) was commonly used to remove the silica template in the
former study [6, 18]. However, it was dangerous and difficult to implement. Silica can
react with alkali solution according to the following reactions:
SiO2  2NaOH  Na2SiO3  H2O
(4)
Reaction products such as Na2SiO3 are easily soluble in water. Thus, silica colloids
sphere can be easily removed by eroding in alkali solutions. Fig. 4 shows the FE-SEM
images and XRD pattern of porous hollow palladium particles after being eroded in
1M NaOH at 60℃. From Fig. 4a and b, Pd particles with hollow structure can be
observed. XRD results reveal that Pd presents a highly crystallized and fcc crystalline
structure (Fig. 4c). The EDS results (inset image in Fig. 4c) show that nearly all of Si
was removed after chemical dealloying. Thus, porous hollow Pd particles with high
purity can be prepared. N2 isotherm curve of porous hollow Pd particles is shown in
Fig. 4d. The porous hollow Pd particles have a high BET area of ~10.26m2/g.
Fig. 5 TEM images of porous hollow palladium particles
Fig. 5 shows TEM image of porous hollow palladium. From Fig. 5a, porous hollow
structure of the resultant palladium particles is further confirmed. Moreover, porous
palladium shells are consisting of palladium nanoparticles (as shown in Fig. 5b),
which is the main reason of high surface area. It is worth to note that this interesting
8

structural feature endows the porous hollow shell and high surface area, which is
especially beneficial for catalysis, hydrogen storage and sensing applications.
Fig. 6 CV curves of porous hollow Pd electrode and Pd plate in 1M KOH solution
deaerated with Ar, the scan rate is 50mV/s
Fig. 6 shows CV curves of porous hollow Pd electrode and Pd plate in 1M
KOH solution. A broad oxidation current peak and a characteristic reduction current
peak (~-0.4V) can be detected. The broad oxidation peak is due to the formation of Pd
surface oxide (PdO), and the corresponding peak is caused by the reduction reaction
took place to form metallic Pd [19-21]. Reduction peak potentials in both materials
are almost the same. However, peak current density of porous hollow palladium is
larger than Pd Plate, which indicated that it possess high electrochemical activity. Pd
electrode composed of hollow structure and high surface area that a greater number of
active sites were available for oxidation. Electrochemical properties of the resultant
porous hollow Pd materials towards ethanol oxidation were also investigated. The
effects of san rate on the CV curves of porous hollow Pd electrode toward ethanol
oxidation were shown in Fig. 7. The peak potential Ep and peak current density jp at
forward scan increased as the increasing of scan rate. Based on Fick diffusion law,
peak current can be obtained according to the equation [22]
3
1
1
2
2
2
ip  269An D Cv
(5)
Where ip is peak current, A is surface area of electrode, D is diffusion coefficient, C is
9

the concentration of electrolyte, and
is scan rate. From Fig. 7b, the peak current
v
were exhibited a linear dependency on the square roots of scan rate, which indicates
that the diffusion control was the main determining factor in alkali condition [22, 23].
Fig. 7 CV curves of porous hollow Pd electrode in the mixture solution of 1M KOH and
0.8M ethanol deaerated with Ar at different scan rates
Fig. 8 CV curves of porous hollow Pd electrode in the mixture solution of 1M KOH
and different concentration of ethanol deaerated with Ar, the scan rate is 50mV/s. The
inset image in Fig. 8a is CV curve of Pd plate in the mixture solution of 1M KOH and
2M ethanol
Concentration dependence of ethanol oxidation on the porous hollow Pd catalyst
electrode was also studied. Fig. 8 shows CVs of porous hollow Pd electrode in the 1
M KOH solution with different ethanol concentrations. It can be found that the jp is
increased as the increasing of ethanol concentrations in the mixing electrolyte below
1M. Afterward, the jp slightly increased when gradually adding ethanol reagents to 2M
(Fig. 8b). Besides, the Ep increased with increasing ethanol concentrations. The
ethanol oxidation reaction on palladium can be expressed as follows [24, 25]:
r.d.s
Pd (CH3CH2OH)ads 3OH Pd (CH3CO)ads 3H2O3e
(6)
r.d.s
Pd (CH3CO)ads  Pd OHads Pd CH3COOH  Pd
(7)
fast


Pd CH3COOH OH Pd CH3COO  H2O
(8)
10

The reactions involve the stripping of the adsorbed ethoxi and releasing the acetate
as the main product. More Pd sites can be occupied by the adsorbed ethoxi at higher
ethanol concentrations. As a contrast, the hydroxyl (OH-) sorption is gradually
suppressed as the ethanol concentration increases. Adsorption of ethoxi will be
dominant in high-concentration ethanol solutions and blocks the adsorption of
hydroxyl. As a result, the insufficient coverage of Pd-OHads will lead to a decrease in
the peak current [24].
The peak current in forward scan of porous hollow palladium is ~380mA/cm2 in
1M KOH and 2M ethanol solution, which is hundreds times than Pd plate in the same
condition (shown in the inset image in Fig. 8). The peak current in forward scan of
porous hollow palladium is also larger than Pd/C (~20mA/cm2) [26], nanoporous
palladium composites (~90 mA/cm2) [20], Porous Pd films (~300mA/cm2)[27] and
ultrafine nanoporous palladium (~150mA/cm2)[28]. The results revealed that superior
ethanol oxidation ability can be obtained with porous hollow palladium as catalyst
materials. Moreover, Nanostructure palladium based materials PdCu [29], PdFe [30],
PdPt [31] and PdNi [32] can greatly enhance the catalytic activity and stability toward
ethanol oxidation. Thus, Pd based porous hollow structure alloyed with Ni, Cu, Au
and Pt can open to window to obtain high electrocatalyst candidate for direct alcohol
fuel cells, which will be involved in the future study.
4. Conclusions
In conclusions, we have successfully prepared porous hollow palladium particles
with high specific surface area of ~10.26m2/g by templating method with silica sphere
11

as template materials. Hydrazine hydrate is a preferable reducing regent to prepare
pure palladium layer. As a contrast, phosphorus impurity can be retained in the
resultant materials by utilizing sodium hypophosphite as reduction. This porous
hollow palladium is expected to act as a promising candidate for the anode catalyst of
direct ethanol fuel cells. In addition, the porous hollow palladium also can be utilized
to investigate their hydrogen isotope storage properties, and which will be discussed
in the future study.
Acknowledgments
The authors would like to acknowledge funding from Ph.D. Programs Foundation
of Ministry of Education of China (Grant no. 20110181110002) and the Scientific and
Technical Project of Sichuan Province (no. 2014GZ0137).
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Fig. 1 FE-SEM images and XRD pattern of silica colloids spheres.
17

Fig. 2 FE-SEM images and XRD patterns of Pd@SiO2 particles after electroless
plating by different reductions (a) Pd@SiO2I, (b) Pd@SiO2II, (c) XRD pattern of (a)
and (b), (d) EDS pattern of Pd@SiO2I, (e) EDS pattern of Pd@SiO2II, respectively.
18

Fig. 3 FE-SEM images of Pd@SiO2Pd@SiO2I particles after electroless plating at
different temperature. (a) 50℃, (b) 60℃, (c) 70℃.
19

Fig. 4 Characterization of porous hollow palladium particles
20

Fig. 5 TEM images of porous hollow palladium particles
21

Fig. 6 CV curves of porous hollow Pd electrode and Pd plate in 1M KOH solution
deaerated with Ar, the scan rate is 50mV/s
22

Fig. 7 CV curves of porous hollow Pd electrode in the mixture solution of 1M KOH and
0.8M ethanol deaerated with Ar at different scan rates
23

Fig. 8 CV curves of porous hollow Pd electrode in the mixture solution of 1M KOH
and different concentration of ethanol deaerated with Ar, the scan rate is 50mV/s. The
inset image in Fig. 8a is CV curve of Pd plate in the mixture solution of 1M KOH and
2M ethanol
24

Highlights
 Silica colloid sphere were fabricated by a modified stöber method.
 Homogeneous porous hollow Pd spheres with high surface area were prepared.
 High purity Pd layer can be obtained with hydrazine hydrate as reduction regent.
 Resultant porous hollow Pd materials show superior catalyst to ethanol oxidation.
25