AgPd nanoparticles supported on zeolitic imidazolate framework derived N-doped porous carbon as an efficient catalyst for formic acid dehydrogenation

Article abstract of DOI:10.1039/c5ra04157k

Formic acid (FA) has tremendous potential as a safe and convenient source of hydrogen for renewable energy storage, but controlled and efficient dehydrogenation of FA by a robust solid catalyst constitutes a major challenge. In this report, a metal nanoparticle functionalized zeolitic imidazolate framework (ZIF) derived N-decorated nanoporous carbon (NPC) support was fabricated and used as an efficient FA decomposition catalyst for the first time. The resultant AgPd@NPC catalyst exhibited 100% H₂ selectivity and high catalytic activity (TOF = 936 h^-1) toward the dehydrogenation of formic acid at 353 K. The synergetic interaction between the metal nanoparticles and NPC greatly enhances the catalytic performance of the as-prepared catalyst.

Full text of DOI:10.1039/c5ra04157k

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AgPd nanoparticles supported on zeolitic
imidazolate framework derived N-doped porous
carbon as an efficient catalyst for formic acid
dehydrogenation†
Cite this: RSC Adv., 2015, 5, 39878
Received 9th March 2015
Accepted 27th April 2015
*
Cheng Feng, Yunhui Hao, Li Zhang, Ningzhao Shang, Shutao Gao, Zhi Wang
DOI: 10.1039/c5ra04157k
*
and Chun Wang
www.rsc.org/advances
Formic acid (FA) has tremendous potential as a safe and convenient distribution infrastructure. Moreover, FA is a promising
source of hydrogen for renewable energy storage, but controlled and candidate as a safe and convenient H2 carrier for portable
efficient dehydrogenation of FA by a robust solid catalyst constitutes a hydrogen storage applications due to its nontoxicity, high
major challenge. In this report, a metal nanoparticle functionalized stability, easy recharging ability, and high energy density.1
zeolitic imidazolate framework (ZIF) derived N-decorated nanoporous The decomposition of formic acid follows two principal
carbon (NPC) support was fabricated and used as an efficient FA pathways: the dehydrogenation pathway to form H2 and CO2
decomposition catalyst for the first time. The resultant AgPd@NPC (eqn (1)), and the dehydration pathway to form H2O and CO
catalyst exhibited 100% H2 selectivity and high catalytic activity (TOF ¼ (eqn (2)).
936 hꢀ1) toward the dehydrogenation of formic acid at 353 K. The
HCOOH(1) / CO2(g) + H2(g) G298 ¼ ꢀ48.8 kJ molꢀ1 (1)
synergetic interaction between the metal nanoparticles and NPC
greatly enhances the catalytic performance of the as-prepared
HCOOH(1) / CO(g) + H2O(g) G298 ¼ ꢀ28.5 kJ molꢀ1 (2)
catalyst.
Reaction (2) is the undesirable pathway as CO is highly toxic
to fuel cell catalysts, so it should be strictly controlled.2 The
reactivity and selectivity of these two pathways are strongly
dependent on the catalysts used, pH of the medium and the
reaction temperature. To achieve efficient FA dehydrogenation,
the development of efficient and cost-effective catalysts is highly
Introduction
With the increased consumption of fossil fuel energy and
growing environmental contamination, the exploitation of
desirable.
clean, renewable and sustainable energies is highly desir-
Up to now, both homogeneous and heterogeneous cata-
able. Among the various alternative energy strategies,
lysts for catalytic FA dehydrogenation have been devel-
hydrogen has been considered as an environmentally
attractive energy carrier for electricity generation in a fuel cell
due to water being the only byproduct.1 However, storage and
distribution of hydrogen in a safe and efficient way still
remain challenging issues.2 Recently, formic acid (FA), one of
the major products of biomass processing and catalytic CO2
hydrogenation with a high H2 content (4.4%), has been
considered as a potential liquid storage material capable of
releasing H2 under mild conditions via catalytic decomposi-
tion. Formic acid as a liquid hydrogen fuel possesses the
potential advantages of utilizing the current liquid-based
oped.3–7 However, the separation and reusability issues
associated with homogeneous catalysts hinder their use in
practical applications. To circumvent these issues, the
development of efficient heterogeneous catalysts has
received considerable attention in recent years.8–13 Pt, Au and
Pd are the primary catalytic metals used in heterogeneous
catalysis.1 Among them, Pt and Au are much more expensive
than Pd. Furthermore, Pt without surface modication or
alloying tends to suffer severe CO poisoning from FA dehy-
dration, and for Au, a high catalytic activity can only be
obtained on subnanometric clusters supported on selected
metal oxides, which is not trival in scale-up synthesis.8 In this
connection, Pd-based nanocatalysts are most attractive for
practical hydrogen production from FA decomposition. In
recent years, heterogeneous catalysts containing Pd,
including Ag–Pd core–shell,10 monodisperse AgPd alloy
nanoparticles11 and metal organic frameworks (MOF)
College of Sciences, Agricultural University of Hebei, Baoding 071001, Hebei, P. R.
China. E-mail: chunwang69@126.com; gst824@163.com; Fax: +86-312-7528292
† Electronic supplementary information (ESI) available: Calculation method of
TOF, the metal mass content in the catalysts, the recyclability of the catalyst,
GC analysis of the evolved gas from aqueous FA solution. See DOI:
10.1039/c5ra04157k
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materials,12,13 have been studied for catalytic dehydrogena- range 2q ¼ 10–90ꢁ. X-ray photoelectron spectroscopy (XPS)
tion of formic acid. It has been demonstrated that the type of was performed with a PHI 1600 spectroscope using Mg Ka X-
the support, the dispersity of the metal nanoparticles and the ray source for excitation. The surface area, total pore volume
synergetic interaction between the metal and support play a and pore size distribution of the samples were measured at
key role for the catalytic performance of the nanocatalysts 77 K by nitrogen adsorption using a APP V-Sorb 2600 Surface
and the kinetic properties of FA dehydrogenation.2
Area and Porosity Analyzer. The metal content of the mate-
Carbons, especially nanoporous carbon materials, have been rials was analyzed by a T.J.A. ICP-9000 type inductively
the most important and traditionally support for heterogeneous coupled plasma atomic emission spectroscopy (ICP-AES)
catalyst due to their high specic surface area and large pore instrument. Detailed analyses for CO2, H2 and CO were per-
volume in combination with excellent thermal, chemical and formed on SP-2000 (Beijing Beifen Ruili Analytical Instru-
mechanical stability. It is well known that the coupling of ment CO., Ltd.). The H2 and CO2 compositions were
nitrogen element into the carbon materials is favorable for the measured by GC spectrum using TCD, while CO was
stabilization of highly dispersed metal nanoparticles.14,15 In measured by GC spectrum using FID-Methanator.
general, N-doped porous carbons (NPC) could be synthesized
via amine etc. graing on the carbon matrix. However, the post Synthesis of NPC
synthetic incorporation method not only blocks pores but also
ZIF-8 was prepared according to the procedure reported by Xu
suffers from leaching and instability in any subsequent regen-
et al.21 Briey, a known quantity (744 mg, 25 mmol) of
eration step. Another synthesis strategy of NPC is high
Zn(NO3)2$6H2O was employed to obtain a mixture with Zn/
temperature pyrolysis of heteroatom-containing precursors. An
MeIM/methanol in a molar ratio of approximately 1 : 1 : 500.
ideal starting material is the one that can act as template,
The mixture was stirred for 1 h at ambient temperature. The
carbon precursor and nitrogen source. In that direction, zeolitic
turbid solution was allowed to stand for 12 h and the solid was
imidazolate framework (ZIF) resulting from periodically
recovered from the milky colloidal solution by centrifugation
arranged organometallic complexes, which has the advantages
and washed with fresh methanol. Then the obtained ZIF-8 was
dried under reduced pressure. For the synthesis of NPC, ZIF-8
nanocrystal (500 mg) was weighed in a ceramic boat and
transferred into a quartz tube. The carbonization of the ZIF-8
was performed at 1073 K, 1173 K and 1273 K, respectively, for
10 h with an argon ow. The resulting product was denoted as
ZIF8-C(n), where n represents the carbonization temperature
of high specic surface area and porosity, chemical tunability,
and well-dened pore structure, might be the promising
precursors and hard templates for in situ casting of NPC since
the organic ligands contain various other types of atoms (N, O,
etc.) other than carbon.16 So far, several ZIFs, such as ZIF-67 (ref.
17, 18), ZIF-7 (ref. 19) and ZIF-8 (ref. 20) have been demon-
strated as precursors to construct nanoporous carbon
(1073, 1173, 1273 K). The Zn residues in ZIF8-C(1073) were
materials.
removed by the HCl solution (10 wt%) which is denoted ZIF8-
In this report, AgPd nanoparticles functionalized with ZIF-8
C(1073)-HCl.
(Zn(MeIM)2, MeIM ¼ 2-methylimidazole) derived NPC would be
fabricated and used as an efficient FA decomposition catalyst
Synthesis of AgPd@NPC
for the rst time. This study demonstrated that the metal
nanoparticles supported on metal–organic frameworks-derived
For preparation of Ag1Pd4@ZIF8-C(1173), 95 mg of ZIF8-C(1173)
was added to 5 mL solution of AgNO3 (0.009 mmol) and newly
NPC materials could act as promising catalysts for the dehy-
prepared H2PdCl4 (0.036 mmol), and stirred for 24 h at 298 K to
drogenation of formic acid.
impregnate the metal salts. Then, ca. 0.4 mL of 2.0 mol Lꢀ1
NaOH solution was added into the above obtained solution with
Experimental
Chemicals
vigorous stirring, followed by an immediate addition of 10 mg
NaBH4 dissolved in 0.50 mL water. The mixture was stirred for 1
h at room temperature. Aer ltration, the obtained Ag1Pd4@
Palladium chloride (PdCl2) and MeIM were all obtained from
ZIF8-C(1173) was dried under reduced pressure. The Ag1-
Aladdin Reagent Limited Company. Formic acid (FA), sodium
Pd4@ZIF8-C(1073), Ag1Pd4@ZIF8-C(1273) were fabricated with
formate (SF), concentrated HCl, Zn(NO3)2$6H2O, AgNO3, NaOH,
the same procedure except that ZIF8-C(1073), ZIF8-C(1273) were
used, respectively. The Pd5@ZIF8-C(1173), Ag2Pd3@ZIF8-
N,N-dimethyl formamide (DMF), methanol and other reagents
were of analytical grade. All the chemicals were used as received
C(1173), Ag5@ZIF8-C(1173) catalysts were synthesized with the
without any purication. High-purity argon and distilled water
above procedure except that the theoretical mass ratios of Ag
and Pd were 0 : 5, 2 : 3 and 5 : 0, respectively. (Scheme 1).
were used in all experiments.
Physical characterizations
General procedure for dehydrogenation of formic acid
reactions
The size and morphology of the nanoparticles were observed
by transmission electron microscopy (TEM) using a JEOL The hydrogen production from FA–SF solution was carried out
model JEM-2011(HR) at 200 kV. The XRD patterns of the in a 10 mL round-bottomed ask, which was placed in an oil
samples were recorded with a Rigaku D/max 2500 X-ray bath with magnetic stirrer at a preset temperature (333–353 K).
diffractometer using Cu Ka radiation (40 kV, 150 mA) in the A gas burette lled with water was connected to the reaction
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Table 1 Textural properties of the Ag1Pd4@ZIF8-C(n) determined by
N2 adsorption–desorption analysis
SBETa (m2 gꢀ1
)
Vb (cm3 gꢀ1
)
dp (nm)
c
Sample
Ag1Pd4@ZIF8-C(1073)
Ag1Pd4@ZIF8-C(1173)
Ag1Pd4@ZIF8-C(1273)
700
466
493
1.32
0.63
0.61
8.71
6.31
5.82
a
b
c
BET surface area. BJH cumulative desorption pore volume. Mean
pore diameter ¼ 4V/SBET
.
Scheme 1 Schematic illustration for the preparation of AgPd@NPC.
that the surface area and pore volume of Ag1Pd4@ZIF8-C(1073),
Ag1Pd4@ZIF8-C(1173) and Ag1Pd4@ZIF8-C(1273) catalysts were
lower than that of the reference value for corresponding NPC,20,21
which could be the result that a certain amount of metals was
deposited in the pores of the NPC during the preparation process.
Fig. 2 displays the XRD pattern of the ZIF8-C(1173),
ask to measure the volume of released gas. Firstly, 26.7 mg
catalyst and 4 mL distilled water were placed in the round-
bottomed ask. Then, the reaction started when 1.0 mL of the
solution containing 1.25 mmol formic acid and 1.25 mmol
sodium formate was injected into the mixture using a syringe.
The molar ratios of Pd/formic acid were theoretically xed at
0.01 for all the catalytic reactions. The volume of the evolved gas
was monitored by recording the displacement of water in the
gas burette.
Pd5@ZIF8-C(1173),
Ag1Pd4@ZIF8-C(1173),
Ag2Pd3@ZIF8-
C(1173), Ag5@ZIF8-C(1173), Ag1Pd4@ZIF8-C(1073) and Ag1-
Pd4@ZIF8-C(1273). In general, the high annealing temperature
allowed the evaporation of zinc (b.p. 1180 K),22 so the zinc
content decreased with increasing the carbonization tempera-
ture from 1073 to 1373 K. The zinc mass content in Ag1Pd4@
ZIF8-C(1073), Ag1Pd4@ZIF8-C(1173) and Ag1Pd4@ZIF8-C(1273)
were 7.49%, 1.69%, and 0.55%, respectively, which were deter-
mined by ICP-AES. Comparing with the corresponding peaks of
Ag1Pd4@ZIF8-C(1073) and Ag1Pd4@ZIF8-C(1273) between
38.03ꢁ and 40.10ꢁ, the peak of Ag1Pd4@ZIF8-C(1173) was
smaller and broader, which suggests that the well dispersion of
AgPd alloy nanoparticles.23,24 The metal mass content in the
catalysts were determined by the ICP-AES, the results were
shown in Table S1 (see ESI,†).
The XPS images of Ag1Pd4@ZIF8-C(1173) were shown in
Fig. 3, the 3d5/2 peak of Pd0 appers at 335.9 eV,10 the 3d5/2 peak
of Ag0 appers at 368.0 eV.10 No obvious peaks of Ag+ and Pd2+
observed, indicating the co-existence of both metals. The 2p2/3
peak of Zn0 apper at 1022.2 eV which is slightly upshied with
respect to the reference value for Zn0,25 indicating that there is a
Results and discussions
Characterization of the catalysts
For the preparation of NPC, the calcination temperature is crucial
in determination of the textural properties and elemental
composition of ZIF8-C(n). The pore properties of the Ag1Pd4@ZIF8-
C(n) catalysts were investigated by nitrogen adsorption–desorption
isotherms (Fig. 1). The steep increase in the adsorbed volume at
low relative pressures reveals the presence of the micropores in the
materials, and the following small slope observed at medium
relative pressures as well as the desorption hysteresis denotes the
existence of the mesopores (curves b and c) while the nal almost
vertical increase at the relative pressures near 1.0 points to the
presence of macropores (curve a). Table 1 summarizes textural
properties of the different annealing temperature catalysts char-
acterized by N2 adsorption–desorption analysis. As Table 1 shows
Fig.
1 N2 adsorption–desorption isotherms of (a) Ag1Pd4@ZIF8-
C(1073); (b) Ag1Pd4@ZIF8-C(1173); (c) Ag1Pd4@ZIF8-C(1273) at 77 K. Fig. 2 Powder X-ray diffraction patterns of AgPd@ZIF8-C(n) and ZIF8-
Filled and open symbols represent adsorption and desorption C(1173). The characteristic peak of Pd (111) is at 2q ¼ 40.10ꢁ, the
branches, respectively.
characteristic peak of Ag (111) is at 2q ¼ 38.03ꢁ.
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and energy-dispersive X-ray spectroscopy (EDX) measurements
(Fig. 4c). A representative high-resolution TEM image in Fig. 4b
shows a d-spacing of 0.227 nm, which is between the (111)
lattice spacing of face-centered cubic (fcc) Ag (0.24 nm) and Pd
(0.22 nm), suggesting that Ag and Pd is formed as an alloy
structure.11 The mean diameter of AgPd nanoparticles in Ag1-
Pd4@ZIF8-C(1173) was in the range of 5–8 nm (Fig. 4a), indi-
cating AgPd nanoparticles was well dispersed on the surface of
NPC.
Dehydrogenation of formic acid reaction
The catalytic dehydrogenation of formic acid has been per-
formed over all the samples at 353 K in the presence of sodium
formate (FAmol : SFmol ¼ 1 : 1) (Fig. 5). Catalytic performances of
all the catalysts were studied based on the amount of gases
measured volumetrically during the reaction. Their catalytic
activities were strongly depended on the composition of AgPd
clusters. The Ag5@ZIF8-C(1173) was almost catalytic inactive.
The catalytic activity increased while by alloying Pd and
increasing the content of Pd. Ag1Pd4@ZIF8-C(1173) exhibits
extremely high catalytic activity with the turnover frequency
(TOF) value of 936 hꢀ1 at 353 K. Further increasing the amount
of Pd to Pd5@ZIF8-C(1173) resulted in the decrease in catalytic
activity, which highlighting the synergistic effect of molecular-
scale AgPd alloy composition in ZIF8-C(1173) for the catalytic
dehydrogenation of FA.
The inuence of calcination temperature on the catalytic
performance of the ZIF8-C(n) catalysts were investigated and the
results were shown in Fig. 6. It can be seen that the volumen of
H2 and CO2 catalyzed by Ag1Pd4@ZIF8-C(1173) reached its
maximum in 5 minutes, which exceed the value of the other
tested catalysts. The results clearly demonstrated that the best
calcination temperature of ZIF-8 was 1173 K. Moreover, when
the NPC ZIF8-C(1173) was treated by 10% HCl for 5 h to remove
the zinc, a slightly lower hydrogen production rate was obtained
with the Ag1Pd4@ZIF8-C(1073)-HCl catalyst. The results clearly
demonstrated that the presence of Zn in NPC may be helpful to
improve the dehydrogenation of FA.
Fig. 3 XPS patterns of the Ag1Pd4@ZIF8-C(1173).
strong interaction between Zn and AgPd alloy. The electron
transfer between Zn and AgPd alloy might lead to the change of
Zn's binding energy. In the growth of ZIF-8 nanoporous
carbons, MeIM ligands act as N source in the annealing process,
thus forming nitrogen-doped carbon. The high resolution N 1s
spectrum (Fig. 3) can be deconvoluted to three sub-peaks due to
the spin orbit coupling, including pyridinic-N (398.5 eV),
pyrrolic-N (399.7 eV) and graphitic-N (401.0 eV), which is a
common characteristic for nitrogen-doped carbon materials.26
In general, nitrogen in carbon texture is favorable for the
stabilization of highly dispersed metal nanoparticles.
The morphologies of Ag1Pd4@ZIF8-C(1173) immobilized
AgPd nanoparticles were characterized by TEM (Fig. 4a and b)
Fig. 4 TEM images of Ag1Pd4@ZIF8-C(1173) with different magnifi- Fig. 5 Gas generation by decomposition of FA with different ratios of
cations (a & b) and EDX spectrum of Ag1Pd4@ZIF8-C(1173) catalyst (c). Ag/Pd supported on ZIF8-C(1173) versus time at 353 K (nFA ¼ nSF ¼ 1.25
The copper signal originates from Cu grid.
mmol, nmetal : nFA ¼ 0.01 : 1).
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Fig. 6 Gas generation by decomposition of FA catalyzed by Ag1-
Pd4@ZIF8-C(1073, 1173, 1273) and Ag1Pd4@ZIF(1073)-HCl catalysts at
353 K (nFA ¼ nSF ¼ 1.25 mmol, nmetal : nFA ¼ 0.01 : 1).
It is known that the decomposition of FA process can be
improved by the addition sodium formate (SF) or NEt3 etc.1 We
studied the hydrogen generation efficiencies of 1 : 1 (molar
ratio) FA–SF, FA and SF solution catalyzed by Ag1Pd4@ZIF8-
C(1173) at 353 K (Fig. 7). The results indicated that 1 : 1 FA–
SF solution exhibits the highest hydrogen generation efficiency
among the three solutions, the turnover frequency (TOF) value
can be reached to 936 hꢀ1. For FA solution, the turnover
frequency (TOF) value was 657 hꢀ1. Moreover, no gas was
generated from pure SF solution.
Except H2, the gas generated from the reaction was only CO2
but no CO has been detected by gas chromatography analysis
(Fig. S1 and S2,†), indicating the excellent H2 selectivity for
formic acid dehydrogenation catalyzed by Ag1Pd4@ZIF8-
C(1173). To get the activation energy (Ea) of the dehydrogena-
tion of FA catalyzed by Ag1Pd4@ZIF8-C(1173), the dehydroge-
nation reactions at different temperature ranging from 323 to
353 K were carried out. The TOF at different temperatures were
calculated from the slope of the linear part of each plot from
Fig. 8a. The Arrhenius plot of ln TOF versus 1/T for the catalyst
was plotted in Fig. 8a, from which the obtained apparent Ea of
the reaction was 23.6 kJ molꢀ1, which is lower than the most
reported values of Ea (Table 2).
Fig. 8 (a) Volume of the generated gas (CO2 + H2) versus time and (b)
corresponding TOF (calculated during the first 5 min of the reactions)
values of H2 generation for the dehydrogenation of FA/SF (1 : 1) at
different temperatures over Ag1Pd4@ZIF8-C(1173). (nFA ¼ nSF ¼ 1.25
mmol, nmetal : nFA ¼ 0.01 : 1, and inset of (a): Arrhenius plot ln TOF vs.
1/T).
reused for at least 5 runs without a signicant loss of the
catalytic activity (Table S2,†), suggesting that the catalyst has a
quite good stability.
Table
2 Comparison of the activities of different catalysts for
hydrogen generation from FA
Catalysts
T (K) TOF (hꢀ1
)
Ea (kJ molꢀ1
)
Ref.
Without additive
Ag/Pd alloy
Ag42Pd58/C
Co0.30Au0.35Pd0.35
Au41Pd59/C
Pd/C
293
323
298
323
323
353
144
382
80
230
30
10
11
29
30
30
33
35
Furthermore, We tested the recyclability of the Ag1Pd4@-
ZIF8-C(1173) catalyst in the dehydrogenation of FA/SF at 353 K.
The result showed that the as-synthesized catalysts could be
22
28
PtRuBiOx/C
AuPd–MnOx/ZIF-8-rGO 298
312
382
37.3
With additive
Ag1Pd4@ZIF8-C(1173)
Ag0.9Pd4.2@ZIF-8
Pd–Au–Eu/C
AuPd@ED-MIL-101
Ni18Ag24Pd58/C
Pd–Au/C
PdAu/C–CeO2
Pd–S–SiO2
Ag0.1–Pd0.9/rGO
PdNi@Pd/GNs-CB
353
353
365
363
323
365
365
358
298
RTa
936
580
387
106
85
23.6
51.4
84.2
This work
12
27
28
31
32
32
34
36
37
20.5
138.6
45
113.5
719
105
577
Fig. 7 Gas generation by decomposition of FA/SF versus time cata-
a
lyzed by Ag1Pd4@ZIF8-C(1173) at 353 K (nFA ¼ 1.25 mmol, nmetal : nFA
¼
RT: room temperature.
0.01 : 1).
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Conclusions
In summary, we have demonstrated the fabrication of in situ
NPC which feature high degree of graphitization, high surface
area and hierarchical porosity by a facile low-cost and readily
reproducible ZIF driven approach. The resultant NPC catalyst,
Ag1Pd4@ZIF8-C(1173), exhibits high catalytic activity for the
catalytic dehydrogenation of formic acid, with the turnover
frequency (TOF) value of 936 hꢀ1 and 100% hydrogen selectivity
at 353 K. Our study reveals that solely ZIF-8 without additional
nitrogen or carbon sources can be used as an ideal precursor to
afford a NPC as catalyst carrier for decomposition of formic
acid, the Zn and nitrogen derived from ZIF-8 nanocrystals can
help to increase the dispersion of the AgPd alloy active species.
With the great number of available and rapidly growing ZIF
structures, ZIF-based NPC materials with tailorable pore
textures and improved performances could be highly
promising.
Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (no. 31171698, 31471643), the
Innovation Research Program of Department of Education of
Hebei for Hebei Provincial Universities (LJRC009), Natural
Science Foundation of Hebei Province (B2011204051,
B2015204003) and the Natural Science Foundation of Agricul-
tural University of Hebei (LG201404).
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J. Q. Zhao, Catal. Commun., 2009, 10, 1454–1458.
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