An efficient CoAuPd/C catalyst for hydrogen generation from formic acid at room temperature

Article abstract of DOI:10.1002/anie.201301009

Less noble: The Co_0.30Au_0.35Pd_0.35 nanoalloy supported on carbon is reported as a stable, low-cost, and highly efficient catalyst for the CO-free hydrogen generation from formic acid dehydrogenation at room temperature (se

Full text of DOI:10.1002/anie.201301009

.
Angewandte
Communications
DOI: 10.1002/anie.201301009
Hydrogen Storage
An Efficient CoAuPd/C Catalyst for Hydrogen Generation from
Formic Acid at Room Temperature**
Zhi-Li Wang, Jun-Min Yan,* Yun Ping, Hong-Li Wang, Wei-Tao Zheng, and Qing Jiang*
Nowadays, searching for the effective hydrogen (H2) storage/
generation materials remains one of the most difficult
challenges toward a fuel-cell-based H2 economy as a long-
term solution for secure energy in future.[1] Formic acid (FA,
HCOOH), a major product of biomass processing with high-
energy density, nontoxicity, and excellent stability at room
temperature, has recently attracted tremendous research
interests for H2 storage and generation.[2,3] Moreover, through
the potential hydrogenation of waste carbon dioxide (CO2)
from industry, FA can be regenerated,[2,4] and this makes the
storage of H2 in FA more attractive for a sustainable and
reversible energy storage cycle.[2,4]
FA can be catalytically decomposed to H2 and CO2
through a dehydrogenation pathway (HCOOH(l)!H2(g) +
CO2(g), DG298K = ꢀ35.0 kJmolꢀ1).[2] However, carbon mon-
oxide (CO), which is a fatal poison to catalysts of fuel cells,[5]
can also be generated through a dehydration pathway
(HCOOH(l)!H2O(l) + CO(g), DG298K = ꢀ14.9 kJmolꢀ1),[2]
depending on the catalysts, pH values of the solutions, as
well as the reaction temperatures.[2,6] Recently, much progress
has been made on the heterogeneous catalysis for the
selective dehydrogenation of FA.[6c–e,7] However, the thermo-
dynamic and kinetic properties of FA dehydrogenation,
especially without any extra additive,[6e,7c] still need to be
further promoted.[6c–e,7,8] More importantly, all the reported
heterogeneous catalysts up to now only consist of noble
metals, including, for example, Pd, Au, Ag, and Pt,[6c–e,7,8]
which greatly hinders their large-scale practical applications
because of their high costs and low reserves in the earthꢀs
crust.
because of their potential activities and relatively low costs.[9]
Whereas, for FA dehydrogenation, nano-FRTM are easily
etched by acidic FA solution. Hence, there is no report on
application of nanocatalyst that includes FRTM for FA
dehydrogenation.[6c–e,7,8]
When FRTM are alloyed with noble metals, their
stabilities under acidic condition can be enhanced, which
depends on the degree of alloying, metallic composition, and
particle size of the material.[10] Moreover, the incorporation of
FRTM into the noble metals with the alloy structure may not
only lead to the enhancement of the catalytic performance,
but also reduce the consumption of the noble metals.[11] In this
sense, a novel strategy to improve the activities and lower the
costs of solid catalysts for FA dehydrogenation is to design the
polymetallic nanomaterials containing FRTM and noble
metals within the stable alloy structures.
Herein, we report the facile synthesis of the CoAuPd
nanoalloy based on a non-noble metal and supported on
carbon (CoAuPd/C) at room temperature (298 K). The
elevated stability of Co0 in the protective nanoalloy structure
makes its first application in FA dehydrogenation successful.
More interestingly, the prepared CoAuPd/C with the lower
consumption of noble metals exhibits the 100% H2 selectivity,
highest activity, and excellent stability toward H2 generation
from FA without any additive at 298 K.
As shown in Scheme 1, CoAuPd/C is synthesized through
a surfactant-free co-reduction method.[12] Typically, for prep-
aration of Co0.30Au0.35Pd0.35/C, 5.0 mL of aqueous solution
containing CoCl2 (9.0 mm), Na2PdCl4 (10.5 mm), and HAuCl4
The first-row transition metals (FRTM) in nanoscale, such
as cobalt (Co) nanoparticles (NPs), have been widely inves-
tigated as the catalytic materials in many important reactions
[*] Z.-L. Wang, Prof. J.-M. Yan, Y. Ping, H.-L. Wang, Prof. W.-T. Zheng,
Prof. Q. Jiang
Key Laboratory of Automobile Materials
Ministry of Education, Department of Materials Science and
Engineering, Jilin University, Changchun 130022 (China)
E-mail: junminyan@jlu.edu.cn
Scheme 1. Preparation and application of CoAuPd/C nanocatalyst for
FA decomposition at 298 K.
jiangq@jlu.edu.cn
(10.5 mm) is mixed with 10.0 mL of aqueous solution con-
taining the well-dispersed Vulcan XC-72 carbon (167.2 mg,
500 m2 gꢀ1).[12] Then, the fresh NaBH4 aqueous solution
(5.0 mL, 300.0 mm) is added to the above mixture under
magnetic stirring in argon (Ar) atmosphere. After 2 h, the
product of Co0.30Au0.35Pd0.35/C is obtained and ready for the
catalytic H2 generation from FA aqueous solution at 298 K.
Figure 1a shows the typical transmission electron micros-
copy (TEM) image of the as-prepared Co0.30Au0.35Pd0.35/C.
The NPs are well-dispersed on carbon with an average
[**] This work is supported in part by the National Natural Science
Foundation of China (grant number 51101070), the National Key
Basic Research, Development Program (grant number
2010CB631001), the Program for New Century Excellent Talents in
University of the Ministry of Education of China (grant number
NCET-09-0431), the Jilin Province Science and Technology Devel-
opment Program (grant number 201101061), and the Jilin Univer-
sity Fundamental Research Funds.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301009.
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(Figure S2). The coexistence of Co, Au, and Pd elements in
the pure fcc phase confirms the formation of
a Co0.30Au0.35Pd0.35 nanoalloy. This can explain that, according
to the Hume–Rothery rule,[14] the relative differences of the
atomic radii of Au (0.144 nm)[15] to Co (0.125 nm)[15] and Pd
(0.138 nm)[15] are (13.2% and 4.2%, respectively) both lower
than 15%, Co and Pd atoms can thus be incorporated into the
Au lattice to form an fcc alloy structure. Compared with fcc
Au, the diffraction peaks of the present fcc phase are slightly
shifted to higher angles (Figure 1g), indicating a little
decrease in the symmetry of the crystal lattice of Au after
addition of Co and Pd, and this is consistent with the HRTEM
result. Based on the above experimental and theoretical
analyses, a Co0.30Au0.35Pd0.35 nanoalloy supported on carbon
has been successfully synthesized through the present facile
co-reduction method at 298 K.
Additionally, the X-ray photoelectron spectroscopy
(XPS) results (Figure 1h,i) show that the binding energies
for Pd 3d and Au 4f in Co0.30Au0.35Pd0.35/C are both shifted to
the lower values compared with those in Pd/C and Au/C,
respectively. Whereas the binding energy for Co 2p in
Co0.30Au0.35Pd0.35/C is shifted to higher value relative to those
in Co/C (Figure 1j). These shifts demonstrate that some
electrons are transferred from Co to Pd and Au atoms in the
[16]
alloy structure of Co0.30Au0.35Pd0.35
.
It can be easily under-
stood that, once Co atoms are closely interacting with Pd and
Au, for instance, in the present alloy structure, electrons can
be transferred from atoms of Co to Pd and Au to equilibrate
the Fermi level because of the difference of the work
functions of Pd(111) (5.67 eV), Au(111) (5.54 eV), and Co(111)
(5.44 eV).[17] Such electron transfer in Co0.30Au0.35Pd0.35/C has
the potential to endow itself with the high activity to H2
generation from FA at 298 K. Fortunately, this assumption
has been confirmed immediately. Moreover, the as-prepared
Co0.30Au0.35Pd0.35/C has good physical and chemical stabilities
in FA solution (see Figures S3 and S4),[12] which makes
possible its application for H2 generation from FA solution.
The catalytic activities of Co0.30Au0.35Pd0.35/C together with
its mono-metallic (Pd/C, Au/C, and Co/C)[12] and bi-metallic
(Au0.50Pd0.50/C, Co0.30Pd0.70/C, and Co0.30Au0.70/C) counter-
parts[12] for H2 generation from FA decomposition at 298 K
in ambient atmosphere are presented in Figure 2. Obviously,
the as-prepared Co0.30Au0.35Pd0.35/C exhibits a much better
activity than those of mono- and bi-metallic catalysts synthe-
sized by the same method.[12] It should be noted that the
decreasing rate in the H2 generation curve over
Co0.30Au0.35Pd0.35/C is due to the reducing FA concentration
during the reaction process but not the deactivation of the
catalyst (Figure S5).[12] Moreover, the reaction rate has a near
linear dependency on the FA concentration with a slope of
0.75 (Figure S6),[12] which further confirms that the reaction
rate is strongly depended on the FA concentration. The
generated gas over Co0.30Au0.35Pd0.35/C is identified by mass
spectrometry (MS, Figure S7) and gas chromatograph (GC,
Figure S8) to be H2 and CO2 with the H2:CO2 molar ratio of
1.0:1.0, and no CO has been detected (detection limit: about
10 ppm, Figure S9). This indicates that CO-free H2 can be
released from FA aqueous solution over the present
Co0.30Au0.35Pd0.35/C catalyst, which is very important for fuel
Figure 1. a) TEM, b) HRTEM, and c) HAADF-STEM images of
Co0.30Au0.35Pd0.35/C and the corresponding elemental mappings for
d) Co, e) Au, and f) Pd elements. g) XRD pattern of Co0.30Au0.35Pd0.35/C.
h) Pd 3d, i) Au 4f, and j) Co 2p XPS spectra for Co0.30Au0.35Pd0.35/C, Pd/
C, Au/C, and Co/C specimens.
particle size of about 20 nm. The corresponding energy-
dispersive X-ray (EDX) spectrum displays all the existences
of Co, Au, and Pd elements (see Figure S1 in the Supporting
Information). And the molar ratio of Co:Au:Pd is determined
to be 0.292:0.357:0.351 by inductively coupled plasma-atomic
emission spectrometry (ICP-AES), which agrees very well
with the appointed value. The high-resolution TEM
(HRTEM) image reveals the crystalline nature of the
Co0.30Au0.35Pd0.35 NPs, and the lattice spacing is measured to
be 0.230 nm (Figure 1b), which is similar to that of the (111)
plane of face-centered cubic (fcc) Au (0.235 nm, JCPDS file:
65-8601).[13] The elemental mappings corresponded to a high-
angle annular dark-field scanning TEM (HAADF-STEM)
image indicating that Co, Au, and Pd are homogeneously
distributed in each particle (Figure 1c–f). The X-ray diffrac-
tion (XRD) pattern shows that the tri-metallic specimen has
the fcc structure of metallic Au (Figure 1g).[13] Moreover, this
fcc structure is stable even after heat treatment at 573 K in Ar
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In summary, the Co0.30Au0.35Pd0.35 nanoalloy supported on
carbon has been successfully applied as a stable and low-cost
catalyst for CO-free H2 generation from FA aqueous solution
at 298 K. The initial TOF and final conversion for the
decomposition of FA can reach the highest values of 80 hꢀ1
and 91% ever reported without any extra additive at room
temperature. This improvement on the catalytic performance
of the nanocatalyst based on a non-noble metal might lead to
a new approach to further develop cost-effective and highly
efficient solid catalysts for the generation of H2 from FA to
meet the requirement of practical application of FA as a H2
storage/generation material.
Figure 2. Gas generation by decomposition of FA (0.5m, 10 mL)
versus time in the presence of a) Co0.30Au0.35Pd0.35/C, b) Co/C, c) Au/C,
Received: February 4, 2013
Published online: March 19, 2013
d) Co0.30Au0.70/C, e) Pd/C, f) Co0.30Pd0.70/C, and g) Au0.50Pd0.50/C (nmetal
FA =0.02) at 298 K in ambient atmosphere.
/
n
Keywords: formic acid · heterogeneous catalysis ·
.
hydrogen storage · nanoalloys · non-noble metals
cell applications.[5,18] The initial turnover frequency [TOF,
Eq. (S1)] over Co0.30Au0.35Pd0.35/C is measured to be 80 hꢀ1 at
298 K. To the best of our knowledge, this initial TOF value is
the highest value ever reported for FA decomposition using
a heterogeneous catalyst without any additive at room
temperature,[6e] and is even comparable to most of those
with additives or at elevated temperatures.[7a–c,g,i] Further-
more, the decomposition of FA can be almost completed
within 600 minutes [91% of conversion, Eq. (S2), Figure 2a].
The initial rate of H2 generation [Eq. (S3)] is determined to be
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ꢀ1
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in the CoAuPd/C system is found to be the most active one for
FA decomposition (Figures S10 and S11).
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compared with tri-metallic Co0.30Au0.35Pd0.35/C (Figure S12),
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