Three-dimensionally ordered mesoporous Pd networks templated by a silica super crystal and their application in formic acid electrooxidation

Article abstract of DOI:10.1039/c1cc11652e

Three-dimensionally ordered mesoporous Pd networks fabricated by a simple reduction method in solution using a face centered cubic silica super crystal as template exhibit high electroactivity in formic acid oxidation.

Full text of DOI:10.1039/c1cc11652e

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Three-dimensionally ordered mesoporous Pd networks templated by a
silica super crystal and their application in formic acid electrooxidationw
Lin Ye,a Yu Wang,a Xueying Chen,*a Bin Yue,a Shik Chi Tsangb and Heyong He*a
Received 22nd March 2011, Accepted 16th May 2011
DOI: 10.1039/c1cc11652e
Three-dimensionally ordered mesoporous Pd networks fabricated
by a simple reduction method in solution using a face centered
cubic silica super crystal as template exhibit high electroactivity
in formic acid oxidation.
hydrazine hydrate (N2H4ꢀH2O) as a reducing agent and the
face centered cubic (fcc) silica super crystal as a template. The
interconnectivity of the Pd replica was ensured by the tight
control of the concentration and volume of Pd precursor
solution for impregnation to fully utilise the interstices in the
silica super crystal and by selecting proper reducing agents to
control the reduction of Pd and growth of Pd networks.
The preparation of the 3DOM Pd networks is as follows:
silica nanoparticles with a uniform diameter of ca. 40 nm were
prepared according to the literature13 with slight modifications.
The details are described in the ESI.w The silica nanoparticles
were self-assembled into a super crystal by centrifugation.
A H2PdCl4 solution (0.56 M) was prepared at 363 K by
dissolving 1.00 g of PdCl2 in 10 ml of hydrochloric acid
(37 wt%) under vigorous stirring. PdCl2 and hydrochloric
acid were both purchased from Sinopharm Group Chemical
Palladium plays a crucial role in organic synthesis,1 H2
sensing2 and many industrial catalytic applications.3 It has
been shown recently that Pd is also promising in direct
alcohol4 or formic acid fuel cells (DFAFCs) applications5–8
which are promising power sources for portable electronics.
Compared with direct methanol fuel cells, DFAFCs can be
operated at a higher voltage and run more efficiently, due to
the unique characteristics of formic acid such as lower cross-
over through the Nafion membrane and the higher kinetic
activity than methanol.6,8 For formic acid oxidation in
DFAFCs, Pd was found to exhibit superior electroactivity to
the Pt catalyst.5 Since the activity of metals strongly depends
on their morphology,9 considerable efforts were made to
prepare metal Pd with well-defined nanostructures.
Reagent Co. Ltd. The dried silica super crystal (0.16 g, SBET =
77 m2 ꢁ1, Vpore = 0.39 cm3 ꢁ1) was then impregnated
g
g
with the above H2PdCl4 solution (0.56 M, 0.11 mmol) with
vigorous stirring by hands for 15 min and dried at 393 K for
12 h. N2H4ꢀH2O (25 wt%, 20 ml) was further added to reduce
Pd2+ and the siliceous template was dissolved by hydrofluoric
acid (20 wt%, 20 ml). The product was centrifuged, washed
with distilled water for 6 times, and methanol for 3 times, and
finally kept in methanol for characterization and activity
test. Electrochemical measurements were performed using a
standard three-electrode electrochemical cell on a CHI660B
electrochemical working station in a 0.5 M H2SO4 + 0.5 M
HCOOH solution (details are described in the ESIw).
The high-resolution scanning electron microscopy
(HRSEM) images (Fig. 1a) and transmission electron
microscopy (TEM) images (Fig. 2a and b) show that a highly
ordered fcc silica super crystal was built from the uniform
silica nanoparticles through centrifugation. After H2PdCl4
impregnation and N2H4ꢀH2O reduction, HRSEM images of
the Pd infiltrated sample (Fig. 1a) demonstrate that the
ordered fcc lattice is well retained during the preparation.
An enlarged HRSEM image (inset in Fig. 1a) shows that Pd
species fill into the interparticle voids of the silica super crystal
template. TEM images (Fig. 2c and d; Fig. S1, ESIw) further
reveal that the Pd is well filled into the interstices of the silica
super crystal template and highly interconnected. After the
removal of the siliceous template, HRSEM (Fig. 1b and c) and
TEM images (Fig. 2e and f) of the silica-free Pd sample
Recently, three-dimensionally ordered mesoporous (3DOM)
precious metals (voids 2–50 nm) with interconnected networks
have shown great potential for a wide range of applications
owing to their fascinating properties inherent to the nature
of the original metal along with high surface areas and ordered
mesostructures.10,11 Although the macroporous metals (voids
450 nm) have been widely practised,12 the fabrication of ordered
mesoporous metals is still challenging due to the narrower
window size. So far, Pt and Au have been mainly reported, but
the fabrication of catalytically active 3DOM Pd networks for fine
chemical synthesis and fuel cells catalysis at milder conditions has
not yet been reported.
Herein, we report the synthesis of 3DOM Pd networks by a
simple, rapid and convenient solution reduction method using
a Department of Chemistry and Shanghai Key Laboratory of
Molecular Catalysis and Innovative Materials, Fudan University,
Shanghai 200433, P. R. China. E-mail: xueyingchen@fudan.edu.cn,
heyonghe@fudan.edu.cn; Fax: +86 21 5566 5572;
Tel: +86 21 6564 3916
b Wolfson Catalysis Centre, Department of Chemistry,
University of Oxford, Oxford, UK, OX1 3QR
w Electronic supplementary information (ESI) available: The details of
silica nanoparticle preparation, the details of Pd electrode preparation
and electrochemical measurements, the TEM images of a Pd infiltrated
silica super crystal, HRTEM and EDX of silica-free 3DOM Pd
networks, and TEM images of the Pt infiltrated silica super crystal.
See DOI: 10.1039/c1cc11652e
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 7389–7391 7389

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Fig. 2 TEM images of (a, b) the silica super crystal template with
corresponding FT patterns and projection charts (insets), (c, d) Pd
infiltrated silica super crystal and (e, f) silica-free 3DOM Pd networks
with an SAED pattern (inset in e) and enlarged image (inset in f). (a, c, e)
were recorded along zone axis h211i, (b, d, f) along zone axis h100i.
parameters, i.e. the procedure of impregnation and type of
reductant, is found to be crucially important. It was found that
continuous 3DOM Pd networks could be easily obtained when
using the near saturated H2PdCl4 precursor solution and the
volume ratio of the H2PdCl4 precursor to the pore of the silica
super crystal template was ca. three. Among N2H4ꢀH2O, SnCl2
and H2, N2H4ꢀH2O was found to be the best reducing agent to
form the continuous Pd networks. Our results reveal that a
relatively fast reduction rate was favorable for metal Pd to
form 3DOM networks with high interconnectivity, which is
quite different from the case of Pt. When N2H4ꢀH2O was
applied for the formation of 3DOM Pt, only Pt nanoparticles
without interconnectivity were formed (Fig. S4, ESIw). The
difference may be due to the slow nucleation12 and slow crystal
growth of Pt.11,14 Compared with the solid reduction method
using dimethylamineborane (DMAB)11 and the H2 reduction
method,12 the present solution reduction method using
N2H4ꢀH2O is more convenient, simple, and non-time-consuming,
which is promising for the further nanostructural control of
metal Pd by the templating technique.
Fig. 1 HRSEM images of (a) Pd infiltrated silica super crystal and
(b) silica-free 3DOM Pd networks. Insets in (a) and (c) are the enlarged
images.
demonstrate that ordered mesoporous Pd networks were well
formed. The pore wall thickness of the 3DOM Pd networks is
ca. 3–8 nm and the mesopore diameter is ca. 40 nm (inset in
Fig. 2f). The high resolution transmission electron microscopy
(HRTEM) image (Fig. S2, ESIw) reveals that the apparent
continuous Pd networks were actually formed by the aggregation
of Pd nanocrystallites. Energy dispersive X-ray emission analysis
(EDX) found that no silica was left in the Pd networks,
verifying the complete removal of the silica template (Fig. S3,
ESIw). The selected area electron diffraction (SAED) pattern
(inset in Fig. 2e) of the 3DOM Pd shows a ring-like pattern
with many intense spots, revealing the polycrystalline nature
of the Pd networks.
Fig. 3a compares the formic acid oxidation activities on the
3DOM Pd networks and ultrafine Pd black electrode catalysts.
The forward peak current for formic acid oxidation on 3DOM
To ensure high interconnectivity of Pd species to form the
Pd networks, controlling the two most important experimental
c
7390 Chem. Commun., 2011, 47, 7389–7391
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enhances the electrochemical stability of Pd electrocatalysts
for the formic acid electrooxidation reaction.
In conclusion, 3DOM Pd networks were successfully
prepared by a simple and convenient solution reduction
method using the interstices in a self-assembled silica super
crystal and their application study for formic acid electro-
oxidation was initially explored. The 3DOM Pd networks
appear to exhibit much better electroactivity than the ultrafine
Pd black, which could be promising as an anodic catalyst for
DFAFCs.
This work was supported by the Ministry of Science and
Technology (2009CB623506, 2009AA033701), the National
Natural Science Foundation (20803009, 20873027), and the
Shanghai Science and Technology Committee (10XD1400300).
We thank Professor Wenbin Cai and Dr Hanxuan Zhang for the
electrochemical measurements and useful discussion.
Notes and references
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 7389–7391 7391