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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: junminyan@jlu.edu.cn (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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