Improved hydrogen production from formic acid under ambient conditions using a PdAu catalyst on a graphene nanosheets-carbon black support

Article abstract of DOI:10.1039/c4ra05379f

Formic acid (FA) has great potential as a suitable liquid source for hydrogen and hydrogen storage materials, provided highly active and selective dehydrogenation catalysts under ambient conditions are developed. Here, well-dispersed bimetallic gold-palladium (PdAu) nanoparticles (NPs) grown on graphene nanosheets-carbon black (GNs-CB) composite supports are synthesized via a facile co-reduction method, wherein the GNs-CB composite support proved to be a powerful dispersion agent and a distinct support for the PdAu NPs. Interestingly, the resultant PdAu/GNs-CB catalyst manifests high selectivity and exceedingly high activity to complete the decomposition of FA at room temperature. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra05379f

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Improved hydrogen production from formic acid
under ambient conditions using a PdAu catalyst on
a graphene nanosheets–carbon black support†
ab
Cite this: RSC Adv., 2014, 4, 30068
Yu-ling Qin,ac Jian-wei Wang,ac Yao-ming Wuab and Li-min Wang
*
Formic acid (FA) has great potential as a suitable liquid source for hydrogen and hydrogen storage materials,
provided highly active and selective dehydrogenation catalysts under ambient conditions are developed.
Here, well-dispersed bimetallic gold–palladium (PdAu) nanoparticles (NPs) grown on graphene
nanosheets–carbon black (GNs–CB) composite supports are synthesized via a facile co-reduction
method, wherein the GNs–CB composite support proved to be a powerful dispersion agent and a
distinct support for the PdAu NPs. Interestingly, the resultant PdAu/GNs–CB catalyst manifests high
selectivity and exceedingly high activity to complete the decomposition of FA at room temperature.
Received 4th March 2014
Accepted 30th June 2014
DOI: 10.1039/c4ra05379f
www.rsc.org/advances
employed to improve the performance of Pd nanocatalysts,
Introduction
which various supports are used to load NPs.5,6 For example, at
ꢀ
elevated temperature (90 C), PdAu NPs loaded on ED-MIL-101
(ref. 5a) or C–CeO2 support,5c and PdAu@Pd/C (ref. 5d) have
been reported to exhibit enhanced catalytic activity. The initial
reaction rꢁa1tes of 106,5a 113.5 (ref. 5c) and 82.6 (ref. 5d) molH2
molcatalyst hꢁ1 have been achieved. To date, highly selective
and efficient PdAu catalysts which can be used to signicantly
improve kinetic properties for catalytic decomposition of FA
under ambient temperature are necessary.
Hydrogen is generally proposed to be an important energy
vector to face the increasing level of energy crisis and environ-
mental pollution due to its high energy density and efficiency
with low environmental load.1 However, even aer several
decades of intensive exploration, hydrogen storage is still one of
the most challenging barriers that impedes the implementation
of the hydrogen-based economy. FA, as a major product of
biomass processing, has attracted considerable attention as a
suitable liquid source for hydrogen and as a potential hydrogen
storage material due to its intrinsic advantages, including high
energy density, nontoxicity, and easy recharging as a liquid (the
availability of the existing infrastructure for gasoline and oil).2
Over the past decades, homogeneous catalysts for FA
decomposition have been intensively investigated and signi-
cant advances have been achieved.3 However, those homoge-
neous catalysts suffer more or less severe drawbacks such as
easy deactivation, hard to separate and recycle, use of organic
solvents, etc.4 To solve these problems, increasing interests have
recently been devoted to heterogeneous catalyst.
Aside from tuning the catalyst composition, adjusting and
optimizing the interaction between the active metal phase and
the support materials are key approaches to further improve
catalytic performance.7 An ideally high performance support
guarantees good dispersion of active catalyst particles, facile
electron transfer, and favorable mass transport kinetics. The
strategy of coupling CB and GNs to form a novel carbon-based
composite support for NPs has been reported in our previous
work.4c Here, we use GNs–CB as a super support to load PdAu
NPs, and graphene oxide (GO) is employed as the precursor of
GNs. The resultant PdAu/GNs–CB catalyst exhibits improved
catalytic performance toward complete decomposition of FA at
room temperature. The prepared PdAu/GNs–CB nanocatalysts
are also used to study the role that sodium formate plays in FA
decomposition reaction.
Pd NPs have been reported to exhibit excellent catalytic
activity toward FA decomposition. However, monometallic Pd is
prone to be deactivated because of the adsorption of poisoning
carbon monoxide intermediates.5 In many works, Au is
aState Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of
Applied Chemistry, Changchun 130022, Jilin, China. E-mail: lmwang@ciac.jl.cn
bChangzhou Institute of Energy Storage Materials&Devices, Changzhou 213000,
Jiangsu, China
Experimental section
Graphene oxide (GO) preparation
GO was made by a modied Hummers method. Briey, graphite
powder (3 g, 325 mesh) was put into an 80 ꢀC solution of
concentrated H2SO4 (12 mL), K2S2O8 (2.5 g), and P2O5 (2.5 g).
cUniversity of Chinese Academy of Sciences, Beijing 100049, China. E-mail: ylqin@
ciac.jl.cn
† Electronic supplementary information (ESI) available: Detailed illustration of
apparatus, TEM and SEM characterization, GC and MS analyses of gas. See DOI:
10.1039/c4ra05379f
ꢀ
Aer keeping at 80 C for 4.5 h using a hot plate, the mixture
was cooled to room temperature and diluted with 0.5 L of
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de-ionized water and le overnight. Then, the mixture was (ICP-AES) measurements were performed on a TJA (Thermo
ltered and washed with de-ionized water using a 0.2 micron Jarrell Ash) Atomscan Advantage instrument.
Nylon Millipore lter to remove the residual acid. The product
was dried under room condition overnight. Next, the pretreated
graphite powder was put into cold (0 C) concentrated H2SO4
(120 mL) in a 250 mL round-bottom ask equipped with a
magnetic stir bar. 15 g KMnO4 was added gradually under
Electrochemical characterizations for the catalysts
ꢀ
For preparation of the thin-lm working electrode, the GC
electrode was sequentially polished with 3 and 0.5 mm Al2O3
paste (mixed with Al2O3 powder and ultrapure water). Aer the
mechanical pretreatment, the electrode was cleaned by soni-
stirring while the temperature of the mixture was kept below
20 ꢀC. The solution was then stirred at 35 ꢀC for 2 h. Aerwards,
cation in ultrapure water. To prepare the working electrode, 5
250 mL of de-ionized water was added and the suspension was
mg of the catalysts was dispersed in diluted Naon alcohol
solution which contained 1000 mL of ethanol and 25 mL of
naon, and was sonicated for 1 h to obtain a uniform suspen-
sion. Next, 8 mL of the suspension was pipetted onto the at
stirred for another 2 h. Subsequently, additional 0.7 L of de-
ionized water was added. Shortly aer that, 20 mL of 30% H2O2
was added to the mixture to destroy the excess of permanganate.
The suspension was then repeatedly centrifuged and washed
glassy carbon electrode. The coated electrode was then dried at
rst with 5% HCl solution and then with water. Exfoliation of
room temperature for 10 min. Electrochemical experiments
graphite oxide to GO was achieved by ultrasonication of the
were carried out in a standard three-electrode cell at room
dispersion for 120 min.
temperature. The working electrode was the thin-lm electrode
with catalysts. Pt foil and Ag/AgCl electrode were used as the
counter and reference electrodes, respectively. All potentials in
Catalyst preparation
For the rst step, 150 mg of Vulcan XC-72 carbon powder was
ultrasonically dispersed in 20 mL of water and subsequently
mixed with 15 mL GO solution (2 mg mLꢁ1), then the mixture
was stirred/sonicated for 6 h. H2PdCl4 (0.05285 M) and specic
atom proportion of gold element (KAuCl4, 0.0273 M) aqueous
solution were added into the mixture, followed by adding 10 mL
of NaBH4 (10 mg mLꢁ1) solution aer 1 h. The mixture was
stirred for another 8 h at room temperature in order to deposit
the NPs onto the support absolutely. Finally, the desired catalyst
from the suspension was centrifuged and washed with the
distilled water and then dried in vacuum at 80 ꢀC overnight. The
synthesises of PdAu/CB and PdAu/GNs were similar with that of
PdAu/GNs–CB. PdAu/GNs was obtained by frieze drying. In this
work, the ratio of CB/GO support in PdAu/GNs–CB or Pd(Au)/
GNs–CB is 5/1, except the samples in the Fig. 6.
this report referred to Ag/AgCl. All electrolyte solutions were
deaerated with high-purity nitrogen for at least 30 min prior to
any measurement.
FA decomposition reaction
Catalytic reactions were carried out at room temperature using a
two-necked round bottom ask with one of the ask openings
connected to a gas burette and the other for the introduction of
FA (Fig. S1†). Catalytic decomposition reaction of FA for the
release of hydrogen (along with carbon dioxide) was initiated by
stirring the mixture of FA solution introduced via a pressure-
equalization funnel and the aqueous suspension of the catalyst
prepared as described above. The volume of reforming gas was
monitored by using the gas burette.
Instrumentation
Results and discussion
Transmission electron microscope (TEM) was performed using Structural characterizations of the PdAu catalyst
a FEI Tecnai G2 S-Twin instrument with a eld emission gun
operating at 200 kV. Mass analysis of the generated gases was
performed using a ThermoStar™ gas analysis system GSD 320
mass spectrometer (detection limit minimum: Faraday < 20
ppm, C-SEM < 1 ppm, 1–100 amu), wherein argon gas is chosen
as cleaning gas. Gas analysis of the generated gases was per-
As shown in Fig. S2,† in contrast to pristine CB (XC-72), the
GO–CB composite support shows better dispersion in water and
facilitates the subsequent synthesis of highly dispersed PdAu
catalyst. The N2 adsorption–desorption isotherms of GO–CB is
shown in Fig. S3,† and the specic surface area is measured to
be 324 m2 gꢁ1. From Table 1, we can nd that the specic
formed using a Techcomp GC 7900 gas chromatography
surface areas for CB and GNS are smaller than the composite
Analyzer, wherein argon gas is chosen as carrying gas. The
support. Specially, for GNs (GO reduced by NaBH4), the specic
detection limit for CO was below 10 ppm. XPS spectra were
surface area reduces terribly because of the aggregation. The
obtained with an ESCALABMKLL X-ray photoelectron spec-
trometer using an Al Ka source. The liquid chromatogram was
obtained by Shimadzu LC-20AB with RI detector (Shimadzu
Table 1 ICP analyses and average size of NPs and BET surface areas
RID-10A), and Aminex HPX-87H column (Bio-Rad, 300 ꢂ 7.8
for the samples
mm), using 0.5 mM H2SO4 as uent at a ow rate of 0.7 mL
minꢁ1 at 323 K. Powder X-ray diffraction (XRD) patterns were
collected on Bruker D8 Focus Powder X-ray diffractometer using
Cu Ka radiation (40 kV, 40 mA). The electrochemical tests were
carried out with a BioLogic VMP3 electrochemical workstation.
Inductively coupled plasma atomic emission spectroscopy
Metal content
(%)
Average size
of NPs (nm)
BET for
Sample
support (m2 gꢁ1
)
PdAu/CB
PdAu/GNs–CB
PdAu/GNs
Au, 4.04; Pd, 0.87
Au, 4.34; Pd, 0.92
Au, 5.98; Pd, 1.25
3.89 ꢃ 0.7
2.79 ꢃ 0.7
4.03 ꢃ 0.8
202
304
37.8
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difference of BET among the three supports may affect the
average size and dispersion of the NPs.
Fig. 1A shows the transmission electron microscopy (TEM)
image of the as-synthesized PdAu/GNs–CB catalysts. CB and
GNs can be differentiated obviously from Fig. 1A (CB is spher-
ical; GNS is sheet like with wrinkle.). Clearly, GNs and CB are
closely associated. As expected, well-dispersed PdAu NPs on
GNs–CB were obtained. In the case of GO or CB alone under
similar synthesis conditions, only larger PdAu NPs can be found
(Table 1, average size of NPs are obtained from TEM micro-
graphs (Fig. 1A and S4†)), highlighting the advantages in
combining GNs and CB. The high-resolution TEM (HRTEM)
image from Fig. 1B shows that the (111) d-spacing of Pd–Au
nanoparticle is smaller than that of pure fcc-Au, but bigger than
that of pure fcc-Pd. As shown in Fig. 2, X-ray diffraction (XRD)
patterns of Au and Pd supported on GNs–CB both are in good
agreement with the face-centered cubic metallic Au (PDF#65-
2870) and Pd (PDF#46-1043), respectively. Because Au scatters
X-rays more effectively than Pd, the diffractive peaks of Pd in
PdAu NPs are hard to be detected. Furthermore, XRD patterns of
bimetallic NPs reveal that all diffraction peaks are situated
between those of Au and Pd, and thus, are reasonably indexed to
PdAu alloy. The inductively coupled plasma atomic emission
spectroscopy (ICP-AES) results show that the Au/Pd ratio of
catalysts prepared with different support are 2.49, 2.54 and 2.57,
respectively, which are almost in agreement with the ratio of
metal precursors.
Fig. 3 XPS spectra of C1s on (A) GO and (B) GO reduced by NaBH4.
signicantly reduced. However, it is worth noting that some
residual oxygen-containing groups can still be found on the
reduced GO, which could improve wettability and accessibility
of aqueous FA solution to the active surface and improve the
performance of the catalyst.8 In addition, Sharma8b also proved
by experimental observations that the presence of residual
oxygen groups on reduced GO plays an important role on the
removal of carbonaceous species from the adjacent sites, which
may be benet for FA decomposition.
PdAu-catalyzed FA decomposition to generate hydrogen
Fig. 4A shows the plots of volume of generated gas (CO2 + H2)
versus the reaction time during dehydrogenation of aqueous FA
solution catalyzed by PdAu NPs loaded on different supports.
The illustration of apparatus for catalytic reaction is shown in
Fig. S1.† FA decomposition or evaporation at room temperature
without catalyst is not evident in Fig. 4A (trace d). From Fig. S5,†
we can nd that the prepared PdAu NPs without support show
the poor catalytic activity toward FA decomposition at room
temperature. Interestingly, the PdAu catalyst prepared in the
presence of GO–CB composite support (a) exhibits enhanced
activity in completing the decomposition of FA within only 30
min at room temperature in aqueous media (trace a and inset,
Fig. 4A). Gas chromatograms (Fig. S6†) and mass spectral
proles (Fig. S7†) show that no detectable amount of CO (<10
ppm) is found in the gas mixture generated from the dehydro-
genation of FA. Completion of the reaction is conrmed by the
amount of released gas [n(H2 + CO2)/nHCOOH ¼ 2.0] and the
liquid chromatogram spectra (Fig. 4B, Sbefore $ 2Saer). The FA
dehydrogenation performance at room temperature on PdAu/
During the reduction process of Pd and Au precursors, GO
could also be reduced by NaBH4. As shown in Fig. 3, C–C bonds
(284.5 eV) become predominant while the peak of C–O is
Fig. 1 (A) TEM and (B) HRTEM images of PdAu/GNs–CB.
Fig. 4 (A) FA decomposition from 5 mL of solution containing 1 M FA
and 1 M sodium formate using 10 mg (0.058 mmol) of PdAu loaded on
three kinds of supports at room temperature. Inset: the expanded view
Fig. 2 XRD patterns for PdAu, Pd and Au NPs loaded on different of a. (B) Liquid chromatogram of FA solution (1 M FA and 1 M sodium
supports.
formate) over PdAu/GNs–CB before and after reaction.
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GNs–CB is much higher than the reported PdAu/ED-MIL-101
(reaction at 90 ꢀC),5a PdAu/C–CeO2 (reaction at 90 ꢀC),5c
PdAu@Pd/C (reaction at 90 ꢀC),5d and PdAu alloy NPs (reaction
at 50 C)5e catalysts. On the contrary, for the PdAu NPs synthe-
ꢀ
sized with CB or GNs alone, only ꢄ80% (trace b, Fig. 4A) and
ꢄ55% (trace c, Fig. 4A) of H2 are released from FA even aer
more than 5 h, which is much worse than that of PdAu catalyst
prepared with GNs–CB. The values of the initial reaction rates
(30 min) on PdAu/C and PdAu/GNs is 79.1 and 58.5 molH2
molcatalystꢁ1 hꢁ1, which is much smaller than that on PdAu/GNs
Fig. 6 Plot of completeness of FA decomposition over PdAu at
different support ratios CB:GNs at 30 min.
ꢁ1
(175 molH2 molcatalyst hꢁ1).
Regarding the improved catalytic performance of FA
decomposition, we believe that PdAu/GNs–CB should have a
much stronger tolerance to CO adsorption. CO stripping vol-
tammetry is an effective method to determine the anti-
poisoning ability of a catalyst toward CO. As shown in Fig. 5A
and B, both the onset potential and the peak area of CO
oxidation for PdAu/GNs–CB are much lower than those of the
PdAu/CB catalyst, indicating that PdAu/GNs–CB catalyst
possesses a strong anti-poisoning capability of CO.5d No visible
CO oxidation peak for PdAu/GNs (Fig. 5C) is observed, revealing
scarce CO absorption on the surface. Furthermore, cyclic vol-
tammograms (CVs) (Fig. 5D) on the catalysts aer reaction are
employed. Clearly, a CO stripping peak can be found from the
CV on PdAu/CB, which could be explained by the fact that CO
from FA decomposition cannot be desorbed from the surface of
the catalyst and thus reducing the activity. As for PdAu/GNs,
although no CO absorbs on the surface of the catalyst, aggre-
gation of GNs (Fig. S8†) decreases the catalytic activity.
shown in Fig. 6. Under the same PdAu loading condition, the
best catalytic activity could be obtained when the ratio of CB to
GNs is 5. Considering the above mentioned results, the poor
activity of PdAu loaded on GNs–CB with other CB/GNs ratios
may be related to the weak dispersion and/or anti-poisoning
ability.
Bimolecular HCOOH/HCOOꢁ mechanism on oxide catalysts
model has been discussed by Borowiak.9 However, Zhou5c
suggests that the mixture containing HCOOH and HCOOꢁ
keeps the NPs in a reduced state, consequently retaining the
stability of the alloy catalyst. In order to elucidate the mecha-
nism HCOONa involved in this dehydrogenation reaction, three
additional experiments were performed in our work. No matter
what decomposition mechanism the FA follows, C–H cleavage is
the rate-determining step in obtaining hydrogen. Therefore, the
concentration of formate ion may also be important for FA
decomposition. To this end, Fig. 7A shows the plots of volume
of generated gas (CO2 + H2) versus the reaction time during FA
dehydrogenation at FA solution with different sodium formate
concentration. Clearly, the reaction rate increases with
increased concentration of formate. In this reaction system,
sodium formate has two functions: increasing the pH value and
the formate ion concentration. Hence, investigating the pH
value separately is essential.
The above mentioned results indicate that the enhanced
activity of PdAu NPs supported on GNs–CB can reasonably be
attributed to the combined effect of good dispersion and small
particle size of PdAu NPs, as well as the anti-poisoning ability of
the catalyst. However, since support plays an important role in
catalytic performance, PdAu loaded on supports with different
ratios of CB/GNs are also prepared. Their catalytic activities are
Fig. 8A shows the plots of volume of generated gas (CO2 + H2)
versus the reaction time during FA dehydrogenation at different
FA solution. Sodium acetate is used to replace sodium formate.
As shown in Fig. 8A, only negligible product gas is observed
from the decomposition of sodium formate solution (trace d,
Fig. 8A). In the case of 2 M FA, only 15% FA is decomposed in
pure FA solution during the rst 60 min (trace b, Fig. 8A, pH ¼
1.87). No FA further decomposes even if the reaction time is
extended to 210 min. For FA–sodium acetate solution (pH ¼
4.02), FA decomposes completely within 3 h (trace a, Fig. 8A) can
be conrmed by liquid chromatography (Fig. 8B). It should be
noted that although sodium acetate is added into 2 M FA
solution aer reaction (60 min) to increase the pH, almost no
improvement in reaction activity is achieved (trace c, Fig. 8A),
indicating that degradation of the catalyst in low pH cannot be
recovered by subsequently increasing the pH. That is, highly
efficient hydrogen production from FA decomposition can be
achieved at room temperature with the help of PdAu/GNs–CB,
provided that pH is sufficiently high. Again, in order to further
conrm this point, FA solution (1 M FA and 1 M sodium
Fig. 5 CO stripping voltammograms on three kinds of catalysts in 0.5
M H2SO4 solution at a scan rate of 50 mV sꢁ1: (A), PdAu/CB; (B), PdAu/
GNs–CB; (C), PdAu/GNs. (D) CVs of three kinds of catalysts after
reaction in 0.5 M H2SO4 solution at a scan rate of 50 mV sꢁ1
.
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Fig. 9 Three repeated testings of GNs–CB supported PdAu (8 mg) in 4
mL of FA solution at room temperature.
during the rst two runs. Unfortunately, the catalytic activity
highly decreases in the third run and forth run (Fig. S9†).
Therefore, further investigations are needed to improve the
durability of PdAu/GNs–CB.
Conclusion
In summary, a facile method is used to synthesize well-
dispersed PdAu NPs grown on a GNs–CB composite support to
combine the advantages of GNs and CB. Unexpectedly, PdAu
NPs loaded on GNs–CB exhibit higher catalytic performance for
FA decomposition at room temperature in aqueous media than
on GNs or CB alone. Furthermore, the as-prepared PdAu NPs
also exhibit high hydrogen selectivity in FA decomposition
reaction that no detectable amount of CO is found in the gas
mixture generated from the dehydrogenation of FA. Sodium
formate also has an important function in promoting decom-
position. This study proposed composite support and could
open an effective method to prepared nanocatalysts for
hydrogen generation from renewable fuels such as FA.
Fig. 7 (A) FA decomposition from 5 mL of solution containing 1 M FA
and different concentration of sodium formate (a, 3 M; b, 1 M; c, 0.5 M;
d, 0.2 M) using 10 mg of PdAu/GNs–CB catalyst. (B) FA decomposition
from 5 mL of solution containing 1 M FA and 1 M sodium formate with
different concentration of HCl (a, 0 M; b, 0.01 M; c, 0.03 M; d, 0.075 M)
using 10 mg of PdAu/GNs–CB catalyst.
Acknowledgements
This research is nancially supported by the National Nature
Science Foundation of China (Grant no. 20111061).
Fig. 8 (A) FA decomposition from 5 mL of solution using 20 mg of
PdAu/GNs–CB. (a, 2 M FA and 2 M sodium acetate; b, 2 M FA; c, 2 M FA
and 2 M sodium acetate (added at the reaction time of 60 min); d, 2 M
sodium formate). (B) Liquid chromatogram of FA solution (a) before
and after reaction.
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