Carbon dioxide mediated, reversible chemical hydrogen storage using a Pd nanocatalyst supported on mesoporous graphitic carbon nitride

Article abstract of DOI:10.1039/c4ta01133c

Reversible, carbon dioxide mediated chemical hydrogen storage was first demonstrated using a heterogeneous Pd catalyst supported on mesoporous graphitic carbon nitride (Pd/mpg-C₃N₄). The Pd nanoparticles were found to be uniformly dispersed onto mpg-C₃N₄ with an average size of 1.7 nm without any agglomeration and further exhibit superior activity for the dehydrogenation of formic acid with a turnover frequency of 144 h^-1 even in the absence of external bases at room temperature. Initial DFT studies suggest that basic sites located at the mpg-C ₃N₄ support play synergetic roles in stabilizing reduced Pd nanoparticles without any surfactant as well as in initiating H ₂-release by deprotonation of formic acid, and these potential interactions were further confirmed by X-ray absorption near edge structure (XANES). Along with dehydrogenation, Pd/mpg-C₃N₄ also proves to catalyze the regeneration of formic acid via CO₂ hydrogenation. The governing factors of CO₂ hydrogenation are further elucidated to increase the quantity of the desired formic acid with high selectivity. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ta01133c

Volume 2 Number 25 7 July 2014 Pages 9415–9902
Journal of
Materials Chemistry A
Materials for energy and sustainability
www.rsc.org/MaterialsA
ISSN 2050-7488
PAPER
Chang Won Yoon et al.
Carbon dioxide mediated, reversible chemical hydrogen storage using
a Pd nanocatalyst supported on mesoporous graphitic carbon nitride

Journal of
Materials Chemistry A
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PAPER
Carbon dioxide mediated, reversible chemical
hydrogen storage using a Pd nanocatalyst
supported on mesoporous graphitic carbon
nitride†
Cite this: J. Mater. Chem. A, 2014, 2,
490
9
a
a
a
ab
ab
Jin Hee Lee, Jaeyune Ryu, Jin Young Kim, Suk-Woo Nam, Jong Hee Han,
ab
c
c
ad
Tae-Hoon Lim, Sanjeev Gautam, Keun Hwa Chae and Chang Won Yoon*
Reversible, carbon dioxide mediated chemical hydrogen storage was first demonstrated using a
heterogeneous Pd catalyst supported on mesoporous graphitic carbon nitride (Pd/mpg-C ). The Pd
nanoparticles were found to be uniformly dispersed onto mpg-C with an average size of 1.7 nm
without any agglomeration and further exhibit superior activity for the dehydrogenation of formic acid
3 4
N
3 4
N
ꢀ1
with a turnover frequency of 144 h even in the absence of external bases at room temperature. Initial
DFT studies suggest that basic sites located at the mpg-C support play synergetic roles in stabilizing
reduced Pd nanoparticles without any surfactant as well as in initiating H -release by deprotonation of
formic acid, and these potential interactions were further confirmed by X-ray absorption near edge
structure (XANES). Along with dehydrogenation, Pd/mpg-C also proves to catalyze the regeneration
of formic acid via CO hydrogenation. The governing factors of CO hydrogenation are further
elucidated to increase the quantity of the desired formic acid with high selectivity.
3 4
N
2
Received 6th March 2014
Accepted 14th April 2014
3 4
N
DOI: 10.1039/c4ta01133c
2
2
www.rsc.org/MaterialsA
To make the storage of hydrogen in a molecular form practical,
it is important that the hydrogen needs to be economically
Introduction
Efficient and sustainable technologies are being extensively transported to a desired site. Since the low transportability of
studied to address urgent concerns about energy and environ- solid materials may compromise high hydrogen storage densi-
mental issues for future energy production and storage. Utili- ties, liquid chemicals seem to offer the greatest potential as
zation of hydrogen via fuel cells is one of the promising hydrogen carriers because the storage and transport of such
alternatives to carbon-based fuels for current power generation. compounds are relatively straightforward.
One of the key issues for achieving hydrogen economy is to
Formic acid (FA), a nontoxic liquid readily obtainable from
5
develop reliable hydrogen storage systems that store large biomass processing, has recently been recognized as a safe and
6
amounts of hydrogen in a secure manner. For the purpose, reversible hydrogen storage material. The chemically stored
1
2
metal hydrides, metal–organic frameworks, and chemical
H
2
from FA can be catalytically liberated even at room temper-
hydrides have been extensively explored as potential hydrogen ature for polymer electrolyte membrane fuel cells (PEMFCs)
storage materials over the last decade. Among these candidates, (HCOOH / CO + H ). From the perspective of H storage, FA
sodium borohydride and ammonia borane have attracted can be regenerated from the hydrogenation of carbon dioxide
3
2
2
2
particular attention as chemical hydrogen storage materials (CO
2
+ H
2
/ HCOOH). The use of CO
2
as a hydrogen carrier
4
since they can release H
2
with high hydrogen storage densities.
(Scheme 1) could be achievable economically in conjunction
a
Fuel Cell Research Center, Korea Institute of Science and Technology, Hwarangno
1
4-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea. E-mail: cwyoon@kist.re.kr
b
Green School, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 136-701, Republic
of Korea
c
Advanced Analysis Center, Korea Institute of Science and Technology, Hwarangno 14-
gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea
d
Department of Clean Energy and Chemical Engineering, Korea University of Science
and Technology, Daejeon, Republic of Korea
†
Electronic supplementary information (ESI) available: Detailed experimental
procedures, TEM, XRD, BET, IR and DFT calculation data. See DOI:
0.1039/c4ta01133c
1
Scheme 1 The CO
2
mediated hydrogen energy cycle.
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6
12a
with the utilization of renewable power sources. Catalyst base-free FA dehydrogenation at room temperature. Nitrogen
design would thus be one of the key processes to realize a CO2- atoms located at mpg-C N were suggested to play dual roles in
3
4
mediated hydrogen energy cycle.
providing adsorption sites for Pd ions to form highly dispersed,
The dehydrogenation of FA has recently been studied using a small-sized Pd nanoparticles without any stabilizing ligands as
7
8
9
number of homogeneous Ru, Ir, and Fe catalysts, as well as well as in enhancing FA deprotonation to accelerate dehydro-
10
11
12
heterogeneous Pd, Au, and Pd-based alloys or core–shell
genation. In addition, this monometallic Pd/mpg-C
3
N
4
catalyst
catalysts. Many of these catalytic systems employed external was also capable of catalyzing CO
2
hydrogenation to yield FA
7,8,9a,10a,11,12c,d
bases to promote gas production from FA.
Speci- with high selectivity.
cally for heterogeneous catalysts, metallic alloy or core–shell
nanoparticles (NPs) have been synthesized to avoid the use of
bases. For example, Tsang et al. reported various M@Pd core–
Results and discussion
shell catalysts (M ¼ metal) and found that Ag@Pd showed the Metallic nanoparticles have shown to possess high activities
highest activity for FA decomposition due to the high electron due to the large surface area and the presence of catalytically
12e
donating ability of Ag. In the same vein, Zhang et al. devel- active sites such as edges and steps. However, large quantities of
oped AgPd alloy catalysts that also exhibited a high initial ligands are typically needed to prevent catalyst aggregation,
ꢀ
1
ꢁ
turnover frequency (TOF) of 382 h at 50 C in the absence of which could produce signicant amounts of chemical wastes
12a
bases. Furthermore, the AuPd/C catalyst for FA dehydroge- during a synthesis procedure. In this study, Pd nanoparticles
12f
nation was also reported by the same research group. More- (Pd NPs) were immobilized onto mpg-C N by stirring of an
3
4
over, combination of Co and PdAu also enabled FA aqueous solution containing Pd(NO ) $2H O (32 mM) in the
3
2
2
12b
13
2+
dehydrogenation without bases.
Very recently, Cai et al.
presence of the support, followed by reducing the adsorbed Pd
designed a Pd-based photocatalyst for FA dehydrogenation. The using H (g) without any additional stabilizing agent (Fig. S1†).
2
authors employed carbon nitride as the semiconductive support The X-ray diffraction (XRD) patterns, pore diameters, and
for photo-assisted H
FA dehydrogenation, only a few catalysts that can promote both the ESI (Fig. S2 and S3; Table S1†). The resulting catalyst
FA dehydrogenation and CO hydrogenation have been repor- contains 9.5 wt% Pd as evidenced by inductively coupled
2
-releases. Despite these steps forward for surface areas of mpg-C N and Pd/mpg-C N are compared in
3 4 3 4
2
ted. One of these catalysts was reported by Hull et al. who plasma mass spectrometry (ICP-MS). The Pd NPs were found to
recently demonstrated the reversibility of CO -based hydrogen be uniformly distributed over the support with an average size
2
8
a
storage, catalyzed by a homogeneous Ir-based system. In of 1.7 nm without any agglomeration, as indicated by high-
addition, Beller and co-workers also showed that a Ru homo- resolution TEM (HRTEM) and scanning transmission electron
geneous catalyst proved to catalyze both FA dehydrogenation microscopy (STEM) (Fig. 1a–d). The highly dispersed, small-
7
c
and CO
2
hydrogenation. However, to the best of our knowl- sized NPs likely resulted from nitrogen atoms located at mpg-
2
+
edge, no viable heterogeneous catalytic system catalyzing both C N via initial intermolecular interactions with the Pd ions;
3
4
directions for reversible catalysis has been established to date.
We report here on Pd NPs supported on mesoporous
graphitic carbon nitride (mpg-C N ) for FA based reversible
3
4
hydrogen storage. This Pd/mpg-C N material was rst
3
4
demonstrated as a catalyst completing the CO
2
-based hydrogen
storage cycle (Scheme 2). The as-developed Pd/mpg-C
3 4
N
material was shown to be a superior catalyst for efficiently
promoting FA dehydrogenation under ambient conditions, and
ꢀ1
afforded a TOF of 144 h , which is comparable to the highest
ꢀ1
value (158 h ) ever reported among heterogeneous catalysts for
Fig. 1 HRTEM analyses: (a and b) images of Pd/mpg-C
catalyzed hydrogen production and HADDF-STEM image of Pd/mpg-C , and (d) particle size distribution
of Pd/mpg-C
3 4
N , (c)
Scheme
storage.
2
Pd/mpg-C N
3 4
3 4
N
3 4
N .
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2
+
i.e., the cationic Pd precursors preferentially interacted with
Controlling factors for the dehydrogenation process were
the nitrogen atoms at mpg-C N . Moreover, the described further identied by varying FA concentration. The rates of H2-
3
4
simple synthetic procedure enabled large-scale preparation release increased as FA concentration increased from 0.50 M to
>3 g) of the catalyst. Upon Pd deposition onto the support, the 3.0 M; however, higher concentrations (>4.0 M) were found to
(
observed graphitic stacking of mpg-C
tained in the XRD pattern at a 2q of 23.5 . However, peaks affording a volcano-shaped curve with the highest value being
3
N
4
was clearly main- have a negative effect on dehydrogenation kinetics (Fig. 3),
ꢁ
ꢀ
1
corresponding to metallic Pd NPs appeared very broadly around 150 h at 3.0 M (Fig. 3). Despite the reduced activities at higher
ꢁ
4
0 , presumably due to the small size of the Pd NPs (Fig. S2†). concentrations of >4.0 M, the catalyst still exhibited enhanced
3 4
Catalytic activity of Pd/mpg-C N for FA dehydrogenation activity compared to previous results even at a high concentra-
was evaluated under ambient conditions. Nearly no promotion tion of 12 M, which likely provides advantages for practical
was observed for the dehydrogenation of an aqueous FA (1.0 M), applications. The rate of H -release was further found to
2
with mpg-C N4 alone (Fig. 2, blue). In contrast, signicant increase as the temperature increased, and the calculated TOF
3
ꢁ
ꢀ1
acceleration with the Pd/mpg-C
even in the absence of any bases. In addition, this catalyst obtained from the rate data gave an apparent activation energy
exhibited a much faster rate of H
commercial, well dispersed Pd/C with the size of ca. 2 nm (10% results ranging from 22 to 49 kJ mol
Pd, Fig. S4†) under the same conditions. These results strongly To gain insight into the potential interactions between Pd
suggest that (i) Pd NPs are catalytically active species and (ii) and C N , we performed density functional theory (DFT)
3 4
N catalyst was clearly found at 55 C reached 324 h (Fig. 4a). Moreover, an Arrhenius plot
ꢀ1
2
-release compared to a of 29.1 kJ mol (Fig. 4b), which is consistent with previous
ꢀ
1
11a,12a,12e
.
3
4
14,15
basic nitrogen atoms located at mpg-C N
help to enhance
3
4
FA deprotonation that was proposed as an important initial step
for FA dehydrogenation. Both the Pd metals and the support
thus played a synergetic role in enhancing the rate of H
from FA. In addition, FA dehydration (HCOOH / CO + H
2
-release
O)
2
which yields CO, detrimental to PEMFCs, as a byproduct did not
found in analysis of gaseous products by FT-IR spectroscopy
(
Fig. S5†). The TOF of the Pd/mpg-C N catalysts was calculated
3 4
ꢀ
1
to be 144 h (initial TOF at 10 min), which is higher than the
recently reported highly active CoPAuPd/C (TOF of 80 h
and comparable to the AgPd/C catalyst (TOF of 158 h ).
ꢀ
1
12b
)
ꢀ
1
12a
Furthermore, our system showed even higher TOF than those
with a recently developed Pd@CN utilizing light during the
13
same period. The TOFs of the recently developed heteroge-
neous catalysts are compared in Table S2.† The observed Fig. 3 Gas production profiles of FA dehydrogenation at various
superior performance of Pd/mpg-C N again likely originated concentrations and a plot of initial TOFs as a function of FA concen-
3
4
trations (inset).
from the synergetic functions of the well-dispersed small-sized
14,15
Pd NPs as well as basic sites contained in mpg-C N .
3
4
Moreover, the Pd/mpg-C
stability, and upon repeated dehydrogenation processes
5 times), no catalytic deactivation occurred (Fig. S6†).
3 4
N catalyst further demonstrated high
(
Fig. 2 Gas production profiles for FA dehydrogenation catalyzed by
Pd/mpg-C
3
N
4
, Pd/C, and mpg-C
3
N
4
. The material (50 mg) was Fig. 4 (a) Gas production profiles of FA dehydrogenation at various
ꢁ
employed to dehydrogenate FA (1.0 M, 10 mL) at 25 C.
temperatures and (b) an Arrhenius plot obtained from the rate data.
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calculations using three melm units (Fig. 5a, red circle). The
resulting optimized structure (1) contained three different types
0
00
of nitrogen sites (N, N , and N ) inside C N (Fig. 5b). The
3
4
optimized geometry showed a corrupted structure with a dihe-
0
ꢁ
dral angle of N–C1–N –C2 of ca. 22 (Fig. S7†), which is consis-
tent with a previous study employing repeated melamine and
16
melem building blocks. Natural bond orbital (NBO) charge
0
analyses revealed that compared to that of N (d; ꢀ0.422 and
2
00
ꢀ
0.429), the sp hybridized N and N sites (d; ꢀ0.517, ꢀ0.525,
and ꢀ0.529) were more negatively charged (Fig. S8†). In addi-
tion, molecular orbital analyses further indicated that HOMOs
2
localized over the sp -hybridized nitrogen atoms located inside
the three melm units (Fig. 5c). Notably, the sp N (or N ) sites
appear to have more diffused orbitals than the N sites, implying
that Pd ions from the precursor or acidic proton of FA likely
interacted with N or N sites. These nitrogen sites may have
induced the formation of small-sized Pd NPs which then stabi-
lized the obtained particles, even in the absence of surfactants.
Moreover, the reduced Pd(0) NPs can likely interact with the
nitrogen atoms at C N , and the electron density of the nitrogen
2
00
0
3 4 3 4
Fig. 6 The XANES spectra of mpg-C N and Pd/mpg-C N , (a) N K-
edge and (b) C K-edge.
2
+
0
0
addition, the photon energies of Pd/mpg-C
3 4
N were found to be
shied into high energies, which also supports the electron
17b
transfer from N into Pd. Likewise, the C K-edge spectra reveal
that the peak intensities associated with carbon species
increased following Pd deposition in both p* and s* regions,
suggesting that C orbital perturbation resulted from electronic
interactions between Pd and C and/or between Pd and N
3
4
atoms could then be transferred into Pd NPs.
To validate this hypothesis, potential interactions between
Pd and N atoms at Pd/mpg-C
X-ray absorption near edge structure (XANES). In the N K-edge
spectra, the sharp and intense p* peaks of mpg-C corre-
3 4
N were further elucidated by
17c,d
(
Fig. 6b).
activity of Pd/mpg-C
the commercial Pd/C catalyst (Fig. 2); i.e., the enhanced charge
density at Pd/mpg-C facilitated the rate of H -release
These results are consistent with the improved
3 4
N
17
3 4
N
for FA dehydrogenation compared to
sponding to the pyridinic and graphitic N species were
observed at 399.2 and 402.1 eV while those of Pd/mpg-C N
3
4
3
N
4
2
appeared at 399.4 and 402.3 eV (Fig. 6a). In the s* region, a
broad peak at >405 eV was observed, which is attributed to C–N
conjugated double bonds (407–410 eV) and/or C–N double
1
2b–e
from FA.
For this catalytic system to be useful for practical applica-
tions, CO hydrogenation should be achieved for reversible
hydrogen storage. Only a few studies related to FA synthesis via
the hydrogenation of CO have been reported using homoge-
17a
2
bonds (411–415 eV). Increases in peak intensities aer Pd
deposition indicate that charge transfer from N sites to Pd
occurred via orbital hybridization between N and Pd. In
17c
2
18
neous and heterogeneous catalysts. In the case of heteroge-
neous catalysts, they still suffered from a low yield and
18f,g
selectivity or from the requirements of high pressure.
material has been employed for CO capture and CO
activation to catalyze chemical reactions such as benzene or
The
C
3
N
4
2
2
19
cyclic olen oxidation and cycloaddition to epoxide. Given
these results, we considered another avenue for the potential
reversibility of FA dehydrogenation by reacting CO2 with H2
3 4 2
using a Pd/mpg-C N catalyst. Since FA can further react with H
to give highly reduced products such as methanol and methane,
triethylamine was employed to trap the formed FA (Scheme 3).
2 2
The total pressure of gases (CO and H ) was regulated to 40 bar,
ꢁ
ꢁ
and upon heating the mixture at 100 C and 150 C, this pres-
sure increased to 51 bar and 61 bar, respectively. The formation
of FA was found to be dependent upon reaction conditions
(
Table 1). In particular, the relative ratio of CO and H inu-
2 2
enced the productivity of FA (entry1–3). For example, a decrease
in CO pressure did not notably affect the quantity of FA
2
Fig. 5 (a) Molecular structure of graphitic carbon nitride (red circle, a
model unit for mpg-C ), (b) a DFT-optimized structure (1) using a
truncated C unit, and (c) the calculated HOMOs of 1.
3 4
N
N
3 4
3 4
Scheme 3 FA synthesis catalyzed by Pd/mpg-C N .
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a
Table 1 CO
2
reduction catalyzed by Pd/mpg-C
3
N
4
reaction conditions for FA synthesis, are now under investiga-
tion. Further improvement of catalytic activity and durability
resulting from this catalyst represents a real opportunity to
b
Pressure (bar)
ꢁ
FA (mmol) attain a CO -mediated, reversible hydrogen storage system to
c
Entry
CO
2
H
2
Temp. ( C)
2
meet future energy requirements.
1
2
3
4
5
6
7
20
13
27
20
13
10
5
20
27
13
20
27
30
35
27
27
100
100
100
150
150
150
150
150
150
1.74
1.70
1.22
3.62
4.74
4.26
3.44
2.05
n.d.
Acknowledgements
This research was funded by the Center of Excellence (COE)
program of the Korea Institute of Science and Technology. A
part of this research was also supported by the Korea Institute of
Energy Technology Evaluation and Planning (KETEP) of the
Ministry of Knowledge Economy (MEST) (the New Renewable
Energy Program, no. 20113030040020).
d
8
9
13
13
e
a
2 3 4
Mixtures of D O (10 mL), triethylamine (2.5 mL) and Pd/mpg-C N
b c 1
(50 mg) were stirred for 24 h. Pressure at 298 K. Determined by H
d
NMR using acetone as an internal standard. 10% commercial Pd/C
was used as a catalyst. mpg-C N was used as a catalyst.
3 4
e
Notes and references
1
(a) P. Adelhelm and P. E. de Jongh, J. Mater. Chem., 2011, 21,
u¨ ttel
(
entry 2), while activity decreased under a lower H
2
pressure,
2417–2427; (b) S.-i. Orimo, Y. Nakamori, J. R. Eliseo, A. Z
and C. M. Jensen, Chem. Rev., 2007, 107, 4111–4132.
although CO
2
pressure was higher (entry 3). These results are
2 (a) K. Sumida, D. St
u¨ ck, L. Mino, J.-D. Chai, E. D. Bloch,
˘
O. Zavorotynska, L. J. Murray, M. Dinca, S. Chavan,
likely due to the enhancement of CO solubility by the added
Et N, and this hypothesis was supported by a C NMR experi-
3
2
1
3
ment that showed the formation of the Et N$CO adduct upon
S. Bordiga, M. Head-Gordon and J. R. Long, J. Am. Chem.
Soc., 2013, 135, 1083–1091; (b) M. Dinc
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3
2
˘
a and J. R. Long,
pressurizing CO
Fig. S9†). The obtained results imply that high pressure of
hydrogen is required to attain high yields. Temperature also
inuenced the FA formation, and upon CO hydrogenation at
2 3 2
in a reactor containing Et N and D O
(
2
ꢁ
1
5
50 C, the FA yield increased signicantly (entry 1 vs. 4 and 2 vs.
). In this preliminary study, the best activity was observed upon
utilization of the CO and H pressures of 13 bar and 27 bar,
2
2
respectively, to give 4.74 mmol of FA (entry 5). Further decreases
in CO pressure, however, showed slightly reduced activities
entry 6–7). Compared to the commercial Pd/C catalyst, an
increased affinity of Pd/mpg-C toward CO resulted in >2
times higher catalytic activity (entry 8). In the absence of the Pd,
no products except Et N$CO were formed (entry 9). Notably, in
all cases, our catalytic system produced only FA without any
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2
(
3
N
4
2
3
2
1
13
byproduct, as evidenced by H and C NMR spectra (Fig. S10†).
5
(a) J. Albert, R. W ¨o lfel, A. B ¨o smann and P. Wasserscheid,
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In conclusion, we established an efficient process for immobi-
lizing Pd NPs on mpg-C N . This green and convenient proce-
3 4
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dure requires no ligands and is easily applicable to large-scale
preparation. The resulting small-sized catalyst, Pd/mpg-C N ,
3
4
showed superior activity for hydrogen production from FA
without any additives under ambient conditions. The obtained
excellent activity likely originated from the large number of
7 (a) B. Loges, A. Boddien, H. Junge and M. Beller, Angew.
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3 4
nitrogen functionalities in mpg-C N , which played key roles as
both stabilizers for Pd nanoparticles as well as basic sites for FA
activation. In addition, the developed catalyst also demon-
strated its capability to synthesize FA selectively with the aid of
triethylamine. The basic sites of the support could also induce
initial interaction with CO for the synthesis of FA. Relevant
2
studies associated with the exploration of plausible mecha-
nisms for FA dehydrogenation, as well as the optimization of
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9
(a) A. Boddien, D. Mellmann, F. G ¨a rtner, R. Jackstell,
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