AgPd nanoparticles supported on MIL-101 as high performance catalysts for catalytic dehydrogenation of formic acid

Article abstract of DOI:10.1039/c4ta02066a

Bimetallic AgPd nanoparticles were successfully immobilized into the metal-organic frameworks (MIL-101), and tested for their catalytic dehydrogenation of formic acid. These catalysts were composition dependent for the catalytic activity. Among all the AgPd@MIL-101 catalysts tested, the Ag ₂₀Pd₈₀@MIL-101 catalyst exhibits the highest catalytic activity for the conversion of formic acid to high-quality hydrogen at 80 °C with a TOF value of 848 h^-1, which is among the highest values reported at 80 °C.

Full text of DOI:10.1039/c4ta02066a

Journal of
Materials Chemistry A
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AgPd nanoparticles supported on MIL-101 as high
performance catalysts for catalytic
dehydrogenation of formic acid†
Cite this: J. Mater. Chem. A, 2014, 2,
1060
1
Received 25th April 2014
Accepted 16th May 2014
a
a
a
c
ab
a
Hongmei Dai, Nan Cao, Lan Yang, Jun Su, Wei Luo* and Gongzhen Cheng
DOI: 10.1039/c4ta02066a
www.rsc.org/MaterialsA
1
Bimetallic AgPd nanoparticles were successfully immobilized into the
metal–organic frameworks (MIL-101), and tested for their catalytic
dehydrogenation of formic acid. These catalysts were composition
dependent for the catalytic activity. Among all the AgPd@MIL-101
catalysts tested, the Ag20Pd80@MIL-101 catalyst exhibits the highest
catalytic activity for the conversion of formic acid to high-quality
HCOOH(l) / CO (g) + H (g), DG ¼ ꢁ48.8 kJ molꢁ (1)
2
2
298
O(l) + CO(g), DG298 ¼ ꢁ28.5 kJ molꢁ (2)
1
HCOOH(l) / H
2
Recently, selective dehydrogenation of formic acid was
observed with soluble catalysts of organometallic complexes
and some insoluble catalysts of noble metals deposited on
10
ꢀ
ꢁ1
hydrogen at 80 C with a TOF value of 848 h , which is among the
11
ꢀ
different supports such as metal oxides and activated carbons.
highest values reported at 80 C.
However, nding the optimal balance state between cost,
selectivity, efficiency and recyclability still remains a consider-
Hydrogen, producing only water as a byproduct, has been able challenge.
considered the most promising solution for alternative energy
On the other hand, due to the high specic surface area and
applications. However, one of the most important application tunable pore size, metal–organic frameworks (MOFs) have
1
2
obstacle is the safe and efficient storage of hydrogen. Various attracted growing attention in the application of gas sorption
12
13
14
hydrogen storage approaches are currently being investigated, and storage, drug delivery, and molecular separation. Given
3
4
including metal hydrides, sorbent materials, and chemical the similarity to zeolites, loading of metal nanoparticles (NPs)
hydride systems. Among them, formic acid, as a major product inside the porous materials of MOFs could afford solid cata-
5
of biomass processing with high hydrogen content (4.4 wt%), lysts. The porous structures of MOFs could restrain the aggre-
high stability, environmental benignity, and easy recharging gation of the metal NPs, and further affect the catalytic activity
ability, has been identied as a safe and convenient hydrogen and recyclability of the catalysts. Loading of metal NPs into the
6
15,16
carrier for portable hydrogen storage application. Moreover, porous structures of MOFs is of current interest.
formic acid is a liquid-phase material which has the potential to
It has been reported that bimetallic NPs can promote the
accept the existing liquid-based distribution infrastructure, catalytic activity and selectivity of the monometallic species. In
which makes formic acid more competitive compared with recent years, Ag–Pd bimetallic NPs including AgPd alloy, Ag–Pd
solid chemical hydrogen materials such as sodium borohydride core–shell, and monodisperse AgPd alloy NPs supported on
(
NaBH
4 3 3
) and ammonia borane (NH BH
) derivatives. Hydrogen carbon based materials have been developed for catalytic
17–21
stored in formic acid could be released via a complete decom- dehydrogenation of formic acid.
However, to the best of our
knowledge, there is currently no report about using Ag–Pd alloy
7,8
2
position method (eqn (1)), making CO as the only by-product.
However, from the perspective of hydrogen storage application, NPs supported on MOFs as catalysts for the dehydrogenation of
9
the undesired reaction pathway (eqn (2)) should be avoided.
formic acid. Herein, we report, for the rst time, the generation
of highly dispersed AgPd NPs immobilized in MIL-101 as a
highly efficient catalyst for dehydrogenation of formic acid.
Compared with other reported catalysts, the Ag Pd @MIL-101
2
0
80
a
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei
ꢁ1
catalyst exhibits the highest activity with a TOF value of 848 h
4
30072, P. R. China. E-mail: wluo@whu.edu.cn; Tel: +86 2768752366
ꢀ
at 80 C, and 100% selectivity for hydrogen generation from
formic acid solution.
b
Suzhou Institute of Wuhan University, Suzhou, Jiangsu, 215123, P. R. China
c
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and
MIL-101 with two cavities of ca. 2.9 and 3.4 nm free diame-
Technology, Wuhan, Hubei, 430072, P. R. China
†
Electronic supplementary information (ESI) available. See DOI: ters accessible through two pore windows of ca. 1.2 and 1.6 nm
1
0.1039/c4ta02066a
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was chosen because of its extra high specic surface area, high
ꢀ
thermal stability (up to 300 C), and high chemical stability in
22
23
water. MIL-101 was synthesized according to the literature.
The un-reacted terephthalic acid in the pores were removed by
solvothermally treating with ethanol and aqueous NH F solu-
4
tion, respectively. The supported AgPd@MIL-101 catalyst was
prepared through solution inltration of activated MIL-101 with
H
2
PdCl
4
and AgNO
3
at pH ¼ 3.0, followed by treatment with
NaBH . The low-angle powder X-ray diffractions (PXRD) of the
4
as-synthesized MIL-101, activated MIL-101, Pd@MIL-101,
Ag@MIL-101, and Ag Pd @MIL-101 exhibit no loss of crys-
2
0
80
tallinity (Fig. 1a), indicating that the integrity of the MIL-101
framework was maintained well during the catalyst preparation.
Furthermore, the wide-angle PXRD pattern of Ag20Pd80@MIL-
101 (Fig. 1b) exhibited a broad peak between the characteristic
peaks for Ag(111) and Pd(111), indicating the formation of the
Ag–Pd alloy. The morphologies of MIL-101 immobilized
Ag Pd NPs were further characterized by transmission elec-
2
0
80
Fig. 2 (a and b) TEM images of Ag20Pd80@MIL-101 reduced by sodium
borohydride; (c and d) TEM images of Ag20Pd80@MIL-101 after five
runs of catalytic hydrolysis of HCOOH.
tron microscopy (TEM) (Fig. 2) and energy-dispersive X-ray
spectroscopy (EDX) measurements (Fig. S1†). The EDX spectra
conrm the presence of Ag–Pd. TEM images of AgPd@MIL-101
indicate that the AgPd NPs are well dispersed, and encapsulated
in the cages of the MIL-101. The mean diameter of AgPd NPs in
AgPd@MIL-101 was in the range of 2.7 ꢂ 0.2 nm (Fig. S2†),
which are small enough to be immobilized into the mesoporous
cavities of MIL-101 (2.9 and 3.4 nm), and big enough to be
limited in the pores of the framework by the windows of MIL-
101 (1.2 and 1.6 nm), resulting in the high catalytic activity and
durability for dehydrogenation of formic acid (vide infra). A
representative high-resolution TEM image in Fig. 2a shows a d-
spacing of 0.232 nm, which is between the (111) lattice spacing
of face-centered cubic (fcc) Ag (0.24 nm) and fcc Pd (0.22 nm),
further suggesting that AgPd is formed as an alloy structure.
The N2 adsorption–desorption isotherms of MIL-101 and
Ag Pd @MIL-101 are shown in Fig. 3. The specic areas of
2
0
80
2
ꢁ1
MIL-101 and Ag Pd @MIL-101 were 2232.80 and 1111.02 m g
respectively. The large decrease in the amount of N
and the pore volume (Fig. S3 and Table S1†) of Ag20Pd80@MIL- Ag20Pd80@MIL-101 at 77 K. Filled and open symbols represent
01 indicates that the cavities of MIL-101 were either occupied
20
80
24
2
adsorption
Fig.
3
N
sorption isotherms of (a) activated MIL-101; (b)
2
2
5
adsorption and desorption branches, respectively.
1
or blocked by the well dispersed AgPd NPs. In the X-ray
Fig. 1 [a and b] Low-angle and wide-angle powder X-ray diffraction patterns of samples: (a) MIL-101; (b) activated MIL-101; (c) Pd@MIL-101; (d)
Ag20Pd80@MIL-101; (e) Ag20Pd80@MIL-101 after five runs of catalytic hydrolysis of HCOOH; (f) Ag@MIL-101. The characteristic peak of Pd(111) is
ꢀ
ꢀ
at 2q ¼ 40.1 , the characteristic peak of Ag(111) is at 2q ¼ 38.03 .
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Table 1 Catalytic activity of catalysts for catalytic hydrolysis of formic
acid
Temp
( C)
TOF
(h
Activation energy
ꢀ
ꢁ1
ꢁ1
Catalyst
)
(kJ mol
)
Ref.
Ag20Pd80@MIL-101
80
848
27
This
work
16
29
30
31
32
21
33
18
18
34
8
AuPd@ED-MIL-101
90
85
92
92
25
50
50
20
20
25
25
92
92
92
106
719
21.4
113.5
80
382
80
144
192
64
Pd–S–SiO
2
PdAu@Au/C
PdAu/C–CeO
2
Co0.30Au0.35Pd0.35
Ag48Pd52
Au/C
Ag/Pd alloy
Ag@Pd/C
Pd/C with citric acid
22
Fig.
different
4
Hydrogen generation from HCOOH in the presence of
catalysts: Ag20Pd80@MIL-101; Ag35Pd65@MIL-101;
Ag48Pd52@MIL-101; Ag63Pd37@MIL-101; Ag78Pd22@MIL-101; Ag@MIL-
PdNi@Pd/GN
Pd–Au-Dy/C
Pd–Au-Ho/C
Pd–Au/C
S
-CB
577
269
224
45
ꢀ
1
01; Pd@MIL-101 at 80 .
98.3
102.1
138.6
35
35
35
5
/2
3/2
3/2
photoelectron spectroscopy (XPS) (Fig. S4†), the 3d and 3d
0
26
5/2
peaks of Pd at 335.2 and 340.5 eV, and the 3d and 3d
0
peaks of Ag at 374.3 and 340.5 eV (ref. 27) were observed,
indicating the co-existence of both metals.
dehydrogenation by the Ag Pd @MIL-101 catalyst. Further-
2
0
80
The composition of the AgPd NPs was tuned by the initial more, as a control experiment, the same amount of Ag Pd
80
2
0
molar ratio of H
2 4 3
PdCl and AgNO and analyzed by inductively NPs and MIL-101 was synthesized and applied to dehydroge-
coupled plasma-atomic emission spectroscopy (ICP-AES) (Table nation of formic acid. As shown in Fig. S8,† for Ag Pd , less
2
0
80
S2†). Ag@MIL-101, Ag Pd @MIL-101, Ag Pd @MIL-101, than 40 mL gas was released over 20 min, and almost no reac-
7
8
22
63
37
Ag Pd @MIL-101, Ag Pd @MIL-101, Ag Pd @MIL-101, tivity was observed for MIL-101 toward hydrogen generation
4
8
52
35
65
20
80
ꢀ
and Pd@MIL-101were synthesized using metal precursors at from formic acid and sodium formate solution at 80 C. These
the Ag/Pd ratios of 5 : 0, 4 : 1, 3 : 2, 1 : 1, 2 : 3, 1 : 4, and 0 : 5, results conrm the synergetic effect of AgPd NPs and the
respectively. The catalytic dehydrogenation of formic acid has framework of MIL-101. To get the activation energy (E ) of the
a
ꢀ
been performed over all the samples at 80 C in the presence of dehydrogenation of formic acid catalyzed by Ag Pd @MIL-
2
0
80
sodium formate as shown in Fig. 4. The catalytic activity was 101, the reactions at different temperatures in the range of 50–
ꢀ
strongly dependent on the Ag–Pd composition, while Ag@MIL- 80 C were carried out. The values of rate constant k at different
101 was catalytically inactive. Among all the bimetallic temperatures were calculated from the slope of the linear part of
AgPd@MIL-101 catalysts investigated, Ag Pd @MIL-101 each plot from Fig. 5a. The Arrhenius plot of ln k vs. 1/T for the
2
0
80
exhibited the highest catalytic activity, with a turnover catalyst is plotted in Fig. 5b, from which the apparent activation
ꢁ
1
ꢁ1
ꢀ
ꢁ1
frequency (TOF) value of 848 molH2 molmetal
which is the highest among all the previously reported catalysts lower than most of the reported values. The durability is impor-
Table 1). Moreover, 140 mg of formic acid can be completely tant for the application of catalysts. We tested the durability of the
converted into H and CO in 15 min only in the presence of 100 Ag Pd @MIL-101 catalyst in the decomposition of formic acid
h
at 80 C, energy was determined to be approximately 27.04 kJ mol , being
(
2
2
2
0
80
ꢀ
mg of the Ag20Pd80@MIL-101 catalyst, while other MIL-101 at 80 C. Subsequent addition of the same amount of formic acid
supported bimetallic AgPd NPs resulted in incomplete dehy- aer the completion of the previous run resulted in no signicant
drogenation of formic acid. These results further indicate the decrease in catalytic activity and no change in the hydrogen
molecular-scale synergy of Ag–Pd alloy NPs. In addition, as a selectivity even aer the h run (Fig. S8†). Fig. 2c and d show a
control experiment, the same amount of Ag20Pd80 NPs and MIL- representative TEM image of Ag Pd @MIL-101 NPs aer the
2
0
80
101 were synthesized and applied to the same reaction h run durability test. As clearly seen from the TEM image,
(Fig. S5†). Almost no reactivity was observed for MIL-101, and there is no noticeable change. Furthermore, from the PXRD in
less than 30 mL gas was generated over 20 min for Ag20Pd80 NPs, Fig. 1, it is observed that the integrity of the MIL-101 framework is
indicating the synergetic effect of Ag-Pd alloy NPs and MIL-101. maintained well during the catalyst test aer the h run.
By selectively removing CO , the gas released during the reac-
In summary, we have developed a facile method for immo-
2
tion was passed through a trap containing 5 M NaOH solution bilizing ultrane AgPd alloy NPs into the frameworks of MIL-
to ensure the complete absorption of CO (Fig. S6†). The volume 101. These catalysts were composition dependent toward
2
ꢀ
of gas passed through NaOH solution was reduced to half, dehydrogenation of formic acid at 80 C. The Ag Pd @MIL-
2
0
80
indicating generation of hydrogen and carbon dioxide 101 catalyst exhibits a marked superiority over its monometallic
completely, with no evidence of CO. Furthermore, only CO but and bimetallic counterparts with different ratios, indicating a
no CO has been detected by gas chromatography (GC) analyses strong molecular-scale synergy of Ag–Pd alloy. Moreover, the
2
28
(
2
20 80
Fig. S7†), indicating the excellent H selectivity for formic acid Ag Pd @MIL-101 catalyst exhibits superior catalytic activity
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Fig. 5 (a) Time plots of total gas volume vs. time for Ag20Pd80@MIL-101 NPs catalyzed hydrolysis of HCOOH at different temperatures; (b)
Arrhenius plot obtained from the data of (a).
and selectivity than Ag20Pd80 NPs without any support, and MIL-
01 without metal NPs, suggesting the synergetic effect of AgPd
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NPs and the framework of MIL-101. The combination of high
activity and selectivity as well as good durability enables
AgPd@MIL-101 to act as a potential catalyst for practical
application in dehydrogenation of formic acid for chemical
hydrogen storage. Furthermore, this simple synthetic method
can be extended to other water stable MOFs as effective
supports to immobilize bimetallic/polymetallic metal NPs for
more applications.
This work was nancially supported by the National Natural
Science Foundation of China (21201134), the Ph. D Programs
Foundation
of
Ministry
of
Education
of
China
(20120141120034), the Natural Science Foundation of Jiangsu
Province (BK20130370), the Natural Science Foundation of
Hubei Province (2013CFB288), Ministry of Science and Tech-
nology of China (2011YQ12003504) and Large-scale Instrument
and Equipment Sharing Foundation of Wuhan University.
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