hydrogen generation from formic acid decomposition at room...
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Hydrogen generation from formic aciddecomposition at room temperature using aNiAuPd alloy nanocatalyst
Zhi-Li Wang, Yun Ping, Jun-Min Yan*, Hong-Li Wang, Qing Jiang
Key Laboratory of Automobile Materials, Ministry of Education, Department of Materials Science and Engineering,
Jilin University, Changchun, Jilin 130022, China
a r t i c l e i n f o
Article history:
Received 4 August 2013
Received in revised form
12 November 2013
Accepted 24 December 2013
Available online 24 February 2014
Keywords:
Heterogeneous catalysis
Formic acid
Alloy nanoparticles
Hydrogen generation
Nickel
* Corresponding author.E-mail address: [email protected] (J.
0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.12.1
a b s t r a c t
Formic acid has been widely regarded as a safe and sustainable hydrogen storage material.
Despite tremendous efforts, developing low-noble-metal-loading material with high ac-
tivity for the dehydrogenation of formic acid remains a great challenge. Here, carbon
supported highly homogeneous trimetallic NiAuPd alloy nanoparticles are prepared and
employed as catalyst for the selective dehydrogenation of formic acid. Unexpectedly, at Ni
molar contents as high as 40%, the resultant Ni0.40Au0.15Pd0.45/C exhibits high activity and
100% hydrogen selectivity for hydrogen generation from formic acid aqueous solution
without any additives even at 298 K. Such a low-noble-metal-loading catalyst with high
activity may greatly encourage the practical application of formic acid as a hydrogen
storage material.
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.
1. Introduction
Hydrogen (H2) has been considered as a promising energy
carrier that may play a key role in power generation in the
future especially coupled with fuel cell technology, wherein
the only product is water [1e4]. However, the release of H2
from renewable sources and its efficient storage remain a
significant challenge toward a H2 based energy economy
[4e6]. Conventional H2 storage methods, such as high-
pressure and cryogenic gas containers have efficiency and
safety issues [7]. Other approaches using liquid organic hy-
drides such as cyclohexane, methylcyclohexane, and decalin
have also been investigated, but high temperature are
-M. Yan).
2013, Hydrogen Energy P48
required to the release of H2 [8e11]. Recently, formic acid
(HCOOH, FA), a major product of biomass processing, is
identified as a potential H2 storage material due to its high
gravimetric energy density, nontoxicity, and can be safely
handled in aqueous solution [12e16]. The release of H2 from
FA through a dehydrogenation pathway (HCOOH (l) / H2
(g) þ CO2 (g) DG298 K ¼ �35.0 KJ mol�1) does not proceed
spontaneously, and suitable catalysts are required [12]. How-
ever, carbon monoxide (CO), which is very toxic to fuel cell
catalysts [17], can also be generated from FA through a
dehydration pathway (HCOOH (l) / H2O (l) þ CO (g)
DG298 K ¼ �14.9 KJ mol�1), and thus should be avoided by
adjusting reaction conditions such as pH values of solution,
reaction temperature, and especially catalyst [12e16].
ublications, LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 8 5 0e4 8 5 6 4851
Recently, there have been several reports of efficient homo-
geneous catalysts catalyzed dehydrogenation of FA for H2
generation at near-ambient temperature [18e20]. For
example, Beller and co-workers have reported a highly active
iron catalyst system for the liberation of H2 from FA at near-
ambient temperature [20]. However, the concerns over the
separation issues with homogeneous catalyst have been the
motivation to develop heterogeneous catalyst with a high
catalytic activity for the dehydrogenation of FA [21].
In response, much progress has been achieved on the
heterogeneous catalysis for the selective dehydrogenation of
FA [21e33]. Precious metals such as Au, Pd, and their bime-
tallic nanoparticles (NPs) are considered to be the most
effective catalysts for this reaction [21e30,32]. For example,
Tsang and colleagues have demonstrated that H2 can be
generated from FA aqueous solution at high rates catalyzed by
Ag@Pd coreeshell NPs without further additive at room tem-
perature [21]. Xu and co-works have employed AuePd/ED-
MIL-101 as a catalyst for the dehydrogenation of FA, and high
activity was observed at 363 K in the presence of sodium
formate [25]. Most recently, Sun and co-works have confirmed
that AgPd alloy NPs supported on Ketjen carbon exhibited
highly active for this reaction at 323 K [30]. However, those
solid catalysts mainly composed of noble metal(s) are not
suitable for large-scale practical application because of their
scarcity and high cost [26]. Therefore, we need to design and
synthesize a new catalyst with reduced precious metal
loading and increased activity. Incorporation of first-row
transition metal into the precious metal structure to form an
alloy or coreeshell structured catalyst is thus highly desirable
but still very challenging.
Herein, we report the synthesis of trimetallic NiAuPd alloy
NPs supported on carbon (denoted as NiAuPd/C) and study its
catalytic activity for the selective dehydrogenation of FA at
room temperature. Ni is chosen as one of the constituents of
the catalyst because of its earth-abundance and low cost [34].
Unexpectedly, in contrast to its mono- and bi-metallic
counterparts, the resultant Ni0.40Au0.15Pd0.45/C exhibits high
activity and 100% H2 selectivity for H2 generation from FA
aqueous solution even without any additives at room
temperature.
2. Experimental methods
2.1. Chemicals
FA (HCOOH, SigmaeAldrich, 96%), sodium tetra-
chloropalladate (II) (Na2PdCl4, Sinopharm Chemical Reagent
Co., Ltd, Pd >36.4%), tetrachloroauric (II) acid (HAuCl4$4H2O,
Sinopharm Chemical Reagent Co., Ltd, Au >47.8%), nickel (Ⅱ)
chloride hexahydrate (NiCl2$6H2O, Sinopharm Chemical Re-
agent Co., Ltd, >98%), iron (II) sulfate heptahydrate (FeS-
O4$7H2O, Sinopharm Chemical Reagent Co., Ltd, >99%),
copper (II) chloride dihydrate (CuCl2$2H2O, Sinopharm
Chemical Reagent Co., Ltd, >99%), sodium borohydride
(NaBH4, Sinopharm Chemical Reagent Co., Ltd, >96%), Vulcan
XC-72 carbon (C, 500 m2 g�1, Sinopharm Chemical Reagent
Co., Ltd), ethanol (C2H5OH, Beijing Chemical Works, >99.7%)
were used without further purification. De-ionized water with
the specific resistance of 18.2 MU cmwas obtained by reversed
osmosis followed by ion-exchange and filtration.
2.2. Synthesis of catalysts
Carbon supported trimetallic NiAuPd alloy NPs with different
molar ratios of Ni:Au:Pdwere synthesized using a coreduction
method without any surfactant at 298 K [35,36]. Typically, for
preparation of Ni0.40Au0.15Pd0.45/C, 5.0 mL of aqueous solution
containing NiCl2 (12.0 mM), HAuCl4 (4.5 mM), and Na2PdCl4(13.5 mM) is mixed with 10.0 mL of aqueous solution con-
taining the well-dispersed carbon (136.3 mg). Then, the fresh
NaBH4 aqueous solution (5.0 mL, 300.0 mM) was dropped into
the above mixture with magnetic stirring (600 r min�1) under
argon atmosphere and stirred for 2 h. The product was sepa-
rated by centrifugation, washed with ethanol for several
times, and dried in vacuum at 298 K.
For comparison, Pd/C (9.94 wt%), Au/C (10.60 wt%), Ni/C
(9.83 wt%), Au0.25Pd0.75/C (9.90 wt%), Ni0.40Au0.60/C (9.81 wt%),
Ni0.40Pd0.60/C (9.78 wt%), Cu0.40Au0.15Pd0.45/C (10.13 wt%), and
Fe0.40Au0.15Pd0.45/C (9.88 wt%) were also prepared by the same
method. The metal loadings for the above catalysts were
measured by inductively coupled plasma-atomic emission
spectroscopy (ICP-AES).
2.3. Characterization
Powder X-ray diffraction (XRD) was performed on a Rigaku
RINT-2000 X-ray diffractometer with Cu Ka. The microstruc-
ture and composition of the specimens were investigated
using a field-emission transmission electron microscope
(TEM, Tecnai F20, Philips) and a field-emission scanning
electron microscope (SEM, JEOL, JSM-6700) equipped with an
energy-dispersive X-ray (EDX) microscopy. ICP-AES measure-
ment was performed on a Thermo Jarrell Ash (TJA) Atomscan
Advantage instrument. Mass spectrometry (MS) analysis for
the generated gas was performed on an OmniStar GSD320
mass spectrometer. Detailed analyses for CO2, H2 and COwere
performed on GC-7900 with thermal conductivity detector
(TCD) and flame ionization detector (FID)-Methanator (detec-
tion limit: w10 ppm for CO).
2.4. Catalytic dehydrogenation of FA for H2 generation
The experimental apparatus of H2 generation from the dehy-
drogenation of FA is shown in Fig. 1. Typically, the as-prepared
Ni0.40Au0.15Pd0.45/C (100.8 mg) was kept in a two-necked
round-bottom flask. One neck was connected to a gas
burette, and the other was connected to a pressure-
equalization funnel to introduce FA aqueous solution (0.5 M,
10.0 mL). The catalytic reaction begun after the FA solution
was added into the flask with magnetic stirring (600 r min�1).
A graduated glass tube filled with water was connected to the
reaction flask to measure the volume of the gas (absolute
volume ¼ observation volume e blank volume) that evolved
from the reaction. The reactor was immersed in a water bath
to stabilize the temperature at 298 K.
The catalytic activities of other catalysts for FA decompo-
sitionwere also applied as the abovemethod. Themolar ratios
of metal:FA (nmetal/nFA) for all the catalytic reactions were kept
Fig. 1 e Experimental apparatus of hydrogen generation
from the dehydrogenation of FA.
Fig. 2 e (a) TEM and (b) HRTEM images of Ni0.40Au0.15Pd0.45/C; (c)
STEM images of Ni0.40Au0.15Pd0.45/C, and the corresponding ele
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 8 5 0e4 8 5 64852
as a constant of 0.02. All experiments were repeated at least
two times. The experiments showed repeatable results.
3. Results and discussion
3.1. Synthesis and characterization ofNi0.40Au0.15Pd0.45/C catalyst
The Ni0.40Au0.15Pd0.45/C is prepared through coreduction of
NiCl2, HAuCl4, and Na2PdCl4 in the presence of carbon using
NaBH4 as a reducing agent [35,36]. The morphology of the as-
prepared Ni0.40Au0.15Pd0.45/C is characterized by TEM and
high-resolution TEM (HRTEM). As shown in Fig. 2(a), it can be
seen clearly that the NPs are well dispersed on carbon support
and the particle size is in the range of 16e35 nm. The HRTEM
EDX and (d) XRD patterns of Ni0.40Au0.15Pd0.45/C; (e) HAADF-
mental mappings for (f) Ni, (g) Au and (h) Pd elements.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 8 5 0e4 8 5 6 4853
image of a single NP (Fig. 2(b)) shows the crystalline nature of
the NP, and the lattice spacing is measured to be 0.228 nm,
which is similar to that of the (111) plane of face-centered
cubic (fcc) Au (0.235 nm, JCPDS file: 65-8601) [37]. The corre-
sponding EDX spectrum (Fig. 2(c)) reveals the existences of Ni,
Au, and Pd elements. The metal loading for the NiAuPd/C is
determined (by ICP-AES) to be 9.91 wt% and the atomic ratio of
Ni:Au:Pd is 0.36:0.18:0.46. The XRD pattern shows in Fig. 2(d)
reveals that the diffraction peaks can be indexed as a face-
centered cubic phase with the crystal planes of (111), (200),
(220), and (311). Compared with that of pure Au (JCPDS file: 65-
8601) [37], the diffraction peaks of the Ni0.40Au0.15Pd0.45 NPs
slightly shift to larger diffractions angles, indicating reduction
of the crystal lattice induced by alloying Ni and Pd with Au
structure to form an alloy structure [31]. Such XRD phenom-
enon has also been observed with other bi- and tri-metallic
alloy NPs [31,38e41].
The formation of the alloy structure is further character-
ized by elemental mapping. Fig. 2(e)e(h) shows the high-angle
annular dark-field scanning TEM (HAADF-STEM) of the
Ni0.40Au0.15Pd0.45 NPs and the related mapping with Ni, Au,
and Pd. It can be seen that the elements of Ni, Au, and Pd are
homogeneously distributed in each particle, indicating the
alloy structure is indeed formed though the present synthesis
method.
3.2. Catalytic dehydrogenation of FA for H2 generation
The catalytic activity of the as-prepared Ni0.40Au0.15Pd0.45/C
toward the dehydrogenation of FA for H2 generation is
examined and compared with its mono- and bi-metallic
counterparts. Fig. 3 shows the volume of gas (CO2 þ H2) gen-
eration versus the reaction time during the H2 generation
from FA aqueous solution (0.5 M, 10.0 mL) in the presence of
different catalysts at 298 K. From these curves, it can be seen
that Pd/C shows a very low activity, with which only 96 mL of
gas is generated within 600 min (Fig. 3(a)), whereas Ni/C and
Au/C show no activity (Fig. 3(b), (c)). The catalytic activities of
the bimetallic Ni0.40Pd0.60/C (Fig. 3(d)) and Au0.25Pd0.75/C
0 100 200 300 400 500 6000
20
40
60
80
Con
vers
ion
(%)
Time (min)
(a) (b) (c) (d) (e) (f) (g) (h)
Fig. 3 e The conversions for the FA decomposition (0.5 M,
10.0 mL) vs time in the presence of (a) Pd/C, (b) Ni/C, (c) Au/
C, (d), Ni0.40Pd0.60/C (e) Au0.25Pd0.75/C, (f) Ni0.40Au0.60/C, (g)
Ni0.40Au0.15Pd0.45/C, and (h) physical mixture of Ni/C, Au/C
and Pd/C (Ni:Au:Pd [ 0.40:0.15:0.45) at 298 K under
ambient atmosphere (nmetal/nFA [ 0.02).
(Fig. 3(e)) can be enhanced by introducing Ni and Au into Pd
structure, respectively, whereas their activities are still very
low. Ni0.40Au0.60/C shows no activity for this reaction may be
attributed to the absence of Pd element (Fig. 3(f)) [31]. Sur-
prisingly, it is found that the co-incorporation of Ni and Au
into the Pd structure to form NiAuPd alloy structure signifi-
cantly enhanced the catalytic activity. As shown in Fig. 3(g),
Ni0.40Au0.15Pd0.45/C exhibits much higher activity than that of
all catalysts prepared in this work, with which 73% of FA can
be converted into H2 and CO2 within 600 min. The initial
turnover frequency (TOF, calculated on the basis of the total
amount of metal) is measured to be 12.4 mol H2 per mol
catalyst per hwithout additive at 298 K. It should be noted that
the degradation in the activities of the catalysts prepared in
this work is due to the reduction FA concentration during the
reaction process, and such a degradation phenomenon have
also been observed in other studies concerned with H2 gen-
eration from FA decomposition without additive [21,30,31,42].
In addition, the generated gas is identified byMS (Fig. 4(a)) and
GC (Fig. 4(b)) to be H2 and CO2 with the H2:CO2 molar ratio of
1.0:1.0, and no CO has been detected (detection limit:
w10 ppm, Fig. 4(c)). Moreover, the H2 selectivity for FA
decomposition does not change during the time periods
studied (Fig. 4(d)), indicating that the present
Ni0.40Au0.15Pd0.45/C promotes the complete dehydrogenation
of FA into H2 and CO2, which is very important for fuel cell
applications [43].
It should be noted that the activity of the present NiAuPd/C
toward the dehydrogenation of FA also strongly depends on
its composition. As show in Fig. 5(a), the volume of the evolved
gas (reaction time: 1 h) increase with increasing Ni molar ratio
(x value) up to 0.40. However, further increase of the Ni con-
tent results in a significant decrease of activity. As a result, the
optimized molar ratio of Ni in Nix(Au0.25Pd0.75)1.0�x/C is 0.40
(x¼ 0.40). On the other hand, the effects of Au and Pd have also
been investigated by changing the molar ratio of Au:Pd in
Ni0.40(AuyPd1.0�y)0.60/C (Fig. 5(b)). As a result, the best Au:Pd
molar ratio is 1:3 (y ¼ 0.15), and too little or too much Au in
Ni0.40(AuyPd1.0�y)0.60/C can result in the much lower activity.
Based on the above results, the Ni0.40Au0.15Pd0.45/C is found to
be the most active one for the dehydrogenation of FA in pre-
sent NiAuPd/C system.
3.3. Discussion
Considering thatmonometallic counterpart of Pd/C shows low
activity for the catalytic dehydrogenation of FA whereas both
Ni/C and Au/C are inactive for this reaction, the enhanced
catalytic activity of the Ni0.40Au0.15Pd0.45/C may be attributed
to the synergistic effect between Ni, Au, and Pd in
Ni0.40Au0.15Pd0.45 alloy NPs. It is reasonable to understand that
the alloying of Ni, Au, and Pd leads to a modification of the
electronic structure of catalysts surface, and thus tunes the
interactions between the catalyst and FA on the catalyst sur-
face, resulting in an enhanced catalytic activity for the dehy-
drogenation of FA [30,31]. This is further confirmed by the fact
that the physical mixture of Ni/C, Au/C, and Pd/C (molar ratio
of Ni:Au:Pd is 0.40:0.15:0.45) for the same reaction exhibits
much lower activity (22.1%, 600 min, Fig. 3(h)) than that of
Ni0.40Au0.15Pd0.45/C.
0 4 8 12 16
Inte
nsity
(a. u
.)
Time (min)
H2
CO2
(b)
0 4 8 12 16
Inte
nsity
(a.u
.)
Time (min)
(1) (2)
CO
CO2
(c)
0 8 16 24 32 40 48
Inte
nsity
(a. u
.)
m (z)
H2
H2O
ArCO2
(a)
0 100 200 300 400 500 6000
20
40
60
80
100
Hyd
roge
n se
lect
ivity
(%)
Time (min)
(d)
Fig. 4 e (a) MS spectrum for the evolved gas from FA aqueous solution (0.5 M, 10 mL) over Ni0.40Au0.15Pd0.45/C at 298 K under
Ar atmosphere; GC spectrum using (b) TCD and (c) FID-Methanator for (1) the evolved gas from FA aqueous solution (0.5 M,
10 mL) over Ni0.40Au0.15Pd0.45/C at 298 K and (2) commercial pure CO; (d) The selectivity for the FA decomposition (0.5 M,
10.0 mL) vs time in the presence of Ni0.40Au0.15Pd0.45/C at 298 K.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 8 5 0e4 8 5 64854
The catalytic activities of the carbon supported Pd-based
trimetallic NPs containing other first-row transition metals
for the dehydrogenation of FA are also investigated (Fig. 6). In
contrast to the high activity of the Ni0.40Au0.15Pd0.45/C, the
replacement of Ni with Fe formation Fe0.40Au0.15Pd0.45/C re-
sults in much lower active for this reaction under analogous
reaction conditions, with which only 34.4% of FA is converted
into H2 and CO2 within 600 min (Fig. 6(b)), whereas
Cu0.40Au0.15Pd0.45/C gives an FA conversion of only 8.2% after
600min (Fig. 6(c)). The above results also demonstrate that the
0.0 0.2 0.4 0.6 0.80
10
20
30
40
50
60
Vol
ume
of g
as (m
L)
x value in Nix(Au0.25Pd0.75)1.0-x/C
(a) (
Fig. 5 e Volume of the gas generation from the dehydrogenation
C with different x value and (b) Ni0.40(AuyPd1.0Ly)0.60/C with diff
nFA [ 0.02, reaction time: 1 h).
synergistic effect between Ni, Au and Pd in the NiAuPd/C
contributes to their highly efficient catalytic performance.
4. Conclusion
In summary, the trimetallic NiAuPd alloy NPs supported on
carbon are prepared and employed as a catalyst for the dehy-
drogenation of FA. The resultant Ni0.40Au0.15Pd0.45/C shows
excellentcatalyticactivity for thedehydrogenationofFAforCO-
0.0 0.2 0.4 0.6 0.8 1.00
10
20
30
40
50
60
Vol
ume
of g
as (m
L)
y value in Ni0.40(AuyPd1.0-y)0.60/C
b)
of FA (0.5 M, 10.0 mL) catalyzed by (a) Nix(Au0.25Pd0.75)1.0Lx/
erent y value at 298 K under ambient atmosphere (nmetal/
0 100 200 300 400 500 6000
20
40
60
80C
onve
rsio
n (%
)
Time (min)
(a) (b) (c)
Fig. 6 e The conversion for the FA decomposition (0.5 M,
10.0 mL) vs time in the presence of (a) Ni0.40Au0.15Pd0.45/C,
(b) Fe0.40Au0.15Pd0.45/C, and (c) Cu0.40Au0.15Pd0.45/C at 298 K
under ambient atmosphere (nmetal/nFA [ 0.02).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 4 8 5 0e4 8 5 6 4855
free H2 generation even without any additives at room tem-
perature. The utilization of first-row transition metal as one of
the components to incorporate noble metal structure to make
an alloy structure and applying it as a catalyst for the dehy-
drogenation of FA may represent a new approach to develop
low-noble-metal-loading and highly efficient solid catalysts for
future practical application of FA as a H2 storage material.
Acknowledgments
This work is supported in part by National Natural Science
Foundation of China (51101070); National Key Basic Research,
Development Program (2010CB631001); Program for New
Century Excellent Talents in University of the Ministry of Ed-
ucation of China (NCET-09-0431); Jilin Province Science and
Technology Development Program (201101061); and Jilin Uni-
versity Fundamental Research Funds.
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