Ag0.1-Pd0.9/rGO: An efficient catalyst for hydrogen generation from formic acid/sodium formate

Article abstract of DOI:10.1039/c3ta12724a

Ag_0.1Pd_0.9 nanoparticles assembled on reduced graphene oxide are synthesized by a facile co-reduction route. The resultant AgPd nanoparticles/reduced graphene oxide exert 100% H₂ selectivity and exceedingly high activity toward the complete decomposition of formic acid at room temperature under ambient conditions.

Full text of DOI:10.1039/c3ta12724a

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Ag0.1-Pd0.9/rGO: an efficient catalyst for hydrogen
generation from formic acid/sodium formate†
Cite this: J. Mater. Chem. A, 2013, 1,
12188
*
Yun Ping, Jun-Min Yan, Zhi-Li Wang, Hong-Li Wang and Qing Jiang
Received 14th July 2013
Ag0.1Pd0.9 nanoparticles assembled on reduced graphene oxide are synthesized by a facile co-reduction
route. The resultant AgPd nanoparticles/reduced graphene oxide exert 100% H2 selectivity and
exceedingly high activity toward the complete decomposition of formic acid at room temperature under
ambient conditions.
Accepted 7th August 2013
DOI: 10.1039/c3ta12724a
www.rsc.org/MaterialsA
Hydrogen (H2), which is widely accepted as a promising energy structure, and potentially low manufacturing costs.18 With these
carrier for transport/mobile applications may play an important advantages, graphene-based nanomaterials are being explore
role in renewable energy technologies in the future.1 However, for use in sensors,19 electronics,20 electrochemical energy
due to the extremely low critical point and very low density of H2 storage,21 efficient catalysts,22 etc. In catalytic studies, its close
gas, efficient storage of H2 remains a bottleneck for the contact with metal NPs is believed to play a signicant role in
H2 based fuel cell economy.2,3 As an alternative, chemical H2 the activity enhancement of the catalyst.13,23 Thus, graphene is a
storage has been attracting considerable attention. Formic acid promising candidate as an ideal substrate to anchor various
(FA, HCOOH), one of the major products formed in biomass functional groups with accessible active sites for high perfor-
processing, is nontoxic and highly stable with a high H2 mance catalysis.24–27
content (4.4%)4 at room temperature, making it a safe and
Herein, a facile co-reduction route is successfully utilized to
convenient H2 carrier in fuel cells for portable use.5 FA can be synthesize Ag0.1Pd0.9 NPs assembled on reduced graphene oxide
decomposed to H2 and CO2 via a dehydrogenation pathway (Ag0.1Pd0.9/rGO), wherein the rGO plays a key role as a powerful
(HCOOH(l) / H2(g)+CO2(g), DG298K ¼ ꢀ35.0 kJ molꢀ1
)
6–8 in the dispersion agent and distinct support for the Ag0.1Pd0.9 NPs.
presence of a suitable catalyst. However, the undesirable The resultant Ag0.1Pd0.9/rGO hybrid exerts 100% H2 selectivity
dehydration pathway (HCOOH(l) / H2O(l)+CO(g), DG298K
¼
and exceedingly high activity toward the complete decomposi-
9–11 to generate carbon monoxide (CO), which is tion of FA at room temperature under ambient conditions.
Graphene oxide (GO) dispersed in water is rstly prepared
ꢀ14.9 kJ molꢀ1
)
a fatal poison to catalysts in fuel cells, should be avoided by
adjusting the catalysts, pH values of the solutions, as well as the using the modied Hummers' method as the graphene
reaction temperatures.12 precursor.28 The Ag0.1Pd0.9/rGO hybrid is synthesized by co-
Metallic particles with ultrane sizes have attracted reduction of GO and the metal precursors of Ag and Pd, as
tremendous research interest because of their unique catalytic shown in Fig. 1. Typically, Ag0.1Pd0.9/rGO is prepared by
properties compared to their bulk counterparts due to the high dispersing GO (3 mL, 10.9 mg mLꢀ1) into 10 mL of water with
surface area and the number of edge and corner atoms.13 ultrasonication for 40 min. Then, 5.0 mL of aqueous solution
However, small particles, especially on the nanoscale, are containing AgNO3 (0.01 mM), Na2PdCl4 (0.09 mM) is added into
frequently subjected to the problem of aggregation, resulting the above suspension with magnetic stirring for 30 min.
from their high surface energy,14 which usually leads to the
serious reduction of their catalytic properties. To avoid this
issue, various types of support materials have been employed to
uniformly disperse the nanoparticles (NPs).15,16 Graphene, a
single-layer carbon material,17 attracts tremendous attention
owing to its high conductivity (103–104 S mꢀ1), huge theoretical
surface area (2600 m2 gꢀ1), unique graphitized basal plane
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
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ta12724a
Fig. 1 Schematic illustration for preparation of AgPd/rGO.
12188 | J. Mater. Chem. A, 2013, 1, 12188–12191
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Subsequently, the above mixture is reduced by a freshly
prepared aqueous solution of NaBH4 (1.05 M, 1.0 mL). Aer 2 h,
the obtained product is washed with water several times and
re-dispersed in 10 mL of water for the catalytic H2 generation
from the FA aqueous solution at 298 K.
The morphologies of the as-prepared Ag0.1Pd0.9/rGO hybrid
and free Ag0.1Pd0.9 NPs are characterized by transmission
electron microscopy (TEM). It can be seen that the as-synthe-
sized Ag0.1Pd0.9 with GO are uniformly dispersed on rGO with
an average particle size of about 6 nm (Fig. 2a and b), while the
NPs prepared without GO are severely aggregated (Fig. 2d),
indicating that rGO lead to the good dispersion of Ag0.1Pd0.9
NPs on its surface. It is widely believed that oxygen-containing
functionalities in GO, such as carboxylic (–COOH), carbonyl
(–C]O), and hydroxy (–OH) groups, are necessary for anchoring
metal ions,29 which thus help to control the sizes and
distributions of the formed metal NPs on the rGO during the
synthetic process.30 The high resolution TEM (HRTEM) image
(Fig. 2c) reveals the crystalline nature of the Ag0.1Pd0.9 NPs, and
the lattice spacing is measured to be 0.230 nm, which is
between the (111) plane of face-centered cubic (fcc) Ag
(0.235 nm)31 and fcc Pd (0.224 nm).32 The X-ray diffraction (XRD)
pattern of the Ag0.1Pd0.9/rGO hybrid and free Ag0.1Pd0.9 (Fig. 3a)
Fig. 3 XRD pattern of (a) Ag0.1Pd0.9/rGO and free Ag0.1Pd0.9 NPs: *AgPd alloy, #
rGO, the C 1s peaks in the XPS spectra of (b) GO and (c) Ag0.1Pd0.9/rGO, XPS
spectra of Pd 3d for (d) the free Ag0.1Pd0.9 and Ag0.1Pd0.9/rGO.
show that the diffraction peaks are located between peaks of the Ag0.1Pd0.9/rGO hybrid. The result for C 1s of the as-
expected from pure Pd (JCPDS: 46-1043)33 and Ag (JCPDS: prepared GO (Fig. 3b) indicates that ve different peaks
04-0783),33 strongly indicating the formation of an alloyed centered at 284.5, 285.6, 286.7, 287.8 and 288.8 eV are observed,
structure. In addition, there is only one sharp peak centered at corresponding to sp2C, sp3C, –C–O, –C]O and –COO groups,
2q ¼ 9.72ꢁ in the XRD pattern of the GO (Fig. S1†), corre- respectively.35 However, aer reduction, the intensities of all the
˚
sponding to a distance of 9.092 A between the stacked GO C 1s peaks binding to oxygen obviously decrease (Fig. 3c),
sheets. Aer reduction, the GO peak disappeared from the XRD revealing that most of the oxygen containing species are
pattern and a broad peak around 25ꢁ emerged, indicating that removed and the majority of the conjugated C networks are
GO has been reduced to rGO.34 Based on the above results, the restored. This conrms the reduction of GO to rGO during the
Ag0.1Pd0.9 NPs with an alloy structure supported on rGO have preparation of the Ag0.1Pd0.9/rGO hybrid, which is consistent
been successfully synthesized by the present method.
with the Raman (Fig. S2†) and ultraviolet visible (UV-Vis,
X-Ray photoelectron spectroscopy (XPS) measurements are Fig. S3†) spectra. On the other hand, elements of Pd and Ag in
performed to determine the compositions and chemical states the nal product are in their metallic states (Fig. 3d, and
S4†).36,37 Moreover, the binding energies of Pd 3d in the
Ag0.1Pd0.9/rGO hybrid shi slightly relative to that in free
Ag0.1Pd0.9 NPs (Fig. 3d), implying that rGO interacts strongly
with the metal NPs. Such interaction may be one of the
promotion effects for the efficient decomposition of FA.
The catalytic activities of the as-prepared Ag0.1Pd0.9/rGO
hybrid, together with the free Ag0.1Pd0.9 NPs and rGO for H2
generation from FA decomposition at 298 K under ambient
atmosphere are presented in Fig. 4. It should be noted that only
the mixture of H2 and CO2 but no CO (detection limit: ꢂ10 ppm
for CO) has been detected by gas chromatography (GC) analyses
(Fig. S5 and S6†), which means that the present Ag0.1Pd0.9/rGO
hybrid has an excellent H2 selectivity for FA dehydrogenation.
Finally, 303 mL of gas, a large volume far exceeding the
theoretical value from FA dehydrogenation (244 mL), can be
generated within 120 min at 298 K. Thereby, the excess of gas is
contributed to H2 generated from hydrolysis of sodium formate
(SF) in this system.11 Considering the gas generated from the
present FA/SF system, the theoretical molar ratio of H2 : CO2
should be 1.66 : 1. However, the measured value by GC is
1.44 : 1. This may result from the small levels of CO2 from
Fig. 2 TEM images with the (a) low, (b) middle and (c) high resolution for
Ag0.1Pd0.9/rGO hybrid, and (d) TEM image for free Ag0.1Pd0.9 NPs.
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efficient catalyst to facilitate the liberation of CO-free H2 from
FA/SF aqueous solution at room temperature. This improve-
ment in the catalytic performance of the new Ag0.1Pd0.9/rGO
composite may further promote the practical application of FA
as a H2 storage material.
Acknowledgements
This work is supported in part by National Key Basic Research,
Development Program (2010CB631001); National Natural
Science Foundation of China (51101070) Program for Chang-
jiang Scholars and Innovative Research Team in University;
Program for New Century Excellent Talents in University of the
Ministry of Education of China (NCET-09-0431); Jilin Province
Science and Technology Development Program (201101061);
and Jilin University Fundamental Research Funds.
Fig. 4 Gas generation by the decomposition of FA/SF (1 M/0.67 M, 5 mL) vs.
time in the presence of (a) Ag0.1Pd0.9/rGO hybrid, (b) free Ag0.1Pd0.9 NPs, (c) rGO
at 298 K under ambient atmosphere (nAgPd/nFA ¼ 0.02).
NaHCO3 in the present acid solution.11 Based on the volume of
H2 (179 mL), the total conversion for the decomposition of the
FA/SF system can reach the value of 87.8% within 120 min.
Furthermore the turnover frequency (TOF) is calculated to be
105.2 mol H2 molꢀ1. catalyst. hꢀ1 at 298 K aer 20 min, which is
faster than that of the most active catalysts ever reported at the
room temperature (ESI Table S1†).6,8,10,11,38 On the other hand,
without rGO, the free Ag0.1Pd0.9 NPs exhibit much lower activity
than that of Ag0.1Pd0.9/rGO hybrid, and only 45 mL of gas is
obtained aer 120 min (Fig. 4b), suggesting that rGO plays a key
role in the decomposition of FA/SF. To further investigate if rGO
has a dramatic effect on the Ag0.1Pd0.9/rGO hybrid, the catalytic
activity of the physical mixture of Ag0.1Pd0.9 and rGO is also
examined. As a result, the physical mixture shows much lower
activity than that of the Ag0.1Pd0.9/rGO hybrid under analogous
conditions (Fig. S7†). Reasonably, the superior catalytic
performance of the Ag0.1Pd0.9/rGO hybrid may be attributed to
the synergistic coupling between Ag0.1Pd0.9 and rGO, which
results from the rGO microstructure, including the defects,
types and surface densities of oxygen-containing functional-
ities.39 Assisted by the favorable microstructure, rGO can lead to
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