Dehydrogenation of Formic Acid at Room Temperature: Boosting Palladium Nanoparticle Efficiency by Coupling with Pyridinic-Nitrogen-Doped Carbon

Article abstract of DOI:10.1002/anie.201605961

The use of formic acid (FA) to produce molecular H₂is a promising means of efficient energy storage in a fuel-cell-based hydrogen economy. To date, there has been a lack of heterogeneous catalyst systems that are sufficiently active, selective, and stable for clean H₂production by FA decomposition at room temperature. For the first time, we report that flexible pyridinic-N-doped carbon hybrids as support materials can significantly boost the efficiency of palladium nanoparticle for H₂generation; this is due to prominent surface electronic modulation. Under mild conditions, the optimized engineered Pd/CN_0.25catalyst exhibited high performance in both FA dehydrogenation (achieving almost full conversion, and a turnover frequency of 5530 h^?1at 25 °C) and the reversible process of CO₂hydrogenation into FA. This system can lead to a full carbon-neutral energy cycle.

Full text of DOI:10.1002/anie.201605961

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201605961
German Edition: DOI: 10.1002/ange.201605961
Hydrogen Storage Materials
Dehydrogenation of Formic Acid at Room Temperature: Boosting
Palladium Nanoparticle Efficiency by Coupling with Pyridinic-
Nitrogen-Doped Carbon
Qing-Yuan Bi, Jian-Dong Lin, Yong-Mei Liu, He-Yong He, Fu-Qiang Huang, and Yong Cao*
Abstract: The use of formic acid (FA) to produce molecular
H2 is a promising means of efficient energy storage in a fuel-
cell-based hydrogen economy. To date, there has been a lack of
heterogeneous catalyst systems that are sufficiently active,
selective, and stable for clean H2 production by FA decom-
position at room temperature. For the first time, we report that
flexible pyridinic-N-doped carbon hybrids as support materi-
als can significantly boost the efficiency of palladium nano-
particle for H2 generation; this is due to prominent surface
electronic modulation. Under mild conditions, the optimized
engineered Pd/CN0.25 catalyst exhibited high performance in
both FA dehydrogenation (achieving almost full conversion,
and a turnover frequency of 5530 hꢀ1 at 258C) and the
reversible process of CO2 hydrogenation into FA. This
system can lead to a full carbon-neutral energy cycle.
For FA to be usable as a H2 carrier, it is essential that only the
first reaction takes place, to achieve the maximum possible H2
formation and to avoid the presence of toxic CO. CO2 is an
ideal C1 building block and hydrogen vector that can be
hydrogenated to form FA (Equation 3).[7]
H2 þ CO2 ! HCOOH
ð3Þ
In this way a full carbon-neutral energy cycle can be obtained.
The dehydrogenation of FA with homogeneous catalysts
has been widely investigated.[8] However, the use of various
additives and/or organic solvents tends to limit their practical
large-scale application. Some of the drawbacks associated
with homogeneous catalysts are largely mitigated in solid
catalysts.[9,10] Among the heterogeneous catalysts, many active
components have been shown to be effective for H2 evolution
by FA decomposition because of the simple handling and
significant reusability associated with heterogeneous materi-
als.[9,10] However, despite their high performance most
reaction processes need the addition of alkali compounds
(sodium or potassium salts, or organic amines), which can
lower the gravimetric energy density of FA.[9a,f–i,10a] The
development of a FA system without any additives would
be advantageous to maximize the overall deliverable capacity
of FA.[9d,e,j–m,10b] This would also be beneficial to the quality of
the H2 gas released. In fact, ultraclean H2 could be used in
direct downstream applications in fuel-cell-based technolo-
gies for clean power generation.[5,10b] For all of these reasons,
it is highly desirable to develop a heterogeneous catalyst that
is simple, efficient, and robust, thereby allowing selective
production of ultrapure H2 gas from FA. Ideally, the catalyst
should work in a base-free aqueous medium under ambient
conditions.
Palladium metal is a common active component in
aqueous FA decomposition at low temperatures and in
base-free conditions.[9e,j] However, the poor H2 selectivity
and intrinsically weak CO tolerance of Pd can lead to reduced
stability of the metal. Hence, tremendous efforts have been
devoted to solving these issues; possible solutions include
alloys and core–shell structures.[9d,k,l] However, the prepara-
tion methods are difficult to control and the catalytic activities
need further improvement. An alternative to this strategy is
to modify the surface electronic structure of the supported Pd
catalyst with underlying flexible materials (for example,
carbon-based supports). A modulated electronic structure of
carbon-supported Pd nanoparticles (NPs) could serve as
a new and effective way to boost efficiency. A crucial feature,
which is still unresolved, is the precise control of surface
properties of the Pd NPs. Herein, for the first time, we present
T
he replacement of conventional fossil fuels with a clean
CO2-neutral energy cycle is a key global challenge.[1] Hydro-
gen is a promising energy carrier that has been largely used in
fuel-cell-based technology.[2] However, its safe storage and
transport remain significant bottlenecks to wider use as
a fuel.[3] Formic acid (HCOOH or FA), is a safe and
convenient hydrogen carrier, which has been extensively
applied in renewable energy storage because of its consid-
erable hydrogen content (4.4 wt%), nontoxicity, liquid state,
easy accessibility, and high stability under ordinary condi-
tions.[2,4] More importantly, the gravimetric energy density of
FA is seven times higher than that of the commercially
available lithium-ion batteries.[5] FA decomposition may
occur by two distinct reaction pathways, either by decarbox-
ylation (Equation 1), or dehydration (Equation 2).[6]
HCOOH ! H2 þ CO2
HCOOH ! H2O þ CO
ð1Þ
ð2Þ
[*] Dr. Q. Y. Bi, J. D. Lin, Dr. Y. M. Liu, Prof. Dr. H. Y. He, Prof. Dr. Y. Cao
Department of Chemistry, Shanghai Key Laboratory of Molecular
Catalysis and Innovative Materials, Collaborative Innovation Center
of Chemistry for Energy Materials, Fudan University
Shanghai 200433 (P.R. China)
E-mail: yongcao@fudan.edu.cn
Dr. Q. Y. Bi, Prof. Dr. F. Q. Huang
State Key Laboratory of High Performance Ceramics and Superfine
Microstructures, Shanghai Institute of Ceramics
Chinese Academy of Sciences
Shanghai 200050 (P.R. China)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201605961.
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an efficient and reliable process involving electron-rich
pyridinic-N addition into the carbon hybrid; we used a chem-
ical method that can effectively modulate the surface
electronic and acid–base properties of the Pd catalyst, and
thus enhance its catalytic performance. In comparison to
existing catalysts, the engineered pyridinic-N-tuned Pd cata-
lyst showed great improvements in terms of catalytic activity
and durability toward FA dehydrogenation and CO2 hydro-
genation.
We began our research by synthesizing a series of CNx
(where x denotes the N/C molar ratio based on elemental
analysis data) with a bio-chitosan-based pyrolysis strategy;
this was done by using a combination of chitosan and
melamine, following the reported procedure.[11] The obtained
materials were then loaded with approximately 10 wt% of Pd
by a wet chemical reduction method.[10b] Awell-defined linear
array of mesoporous structures and nanosheet-like morphol-
ogy were observed for materials with the formulae CN0.15
,
CN0.23, CN0.39, CN0.62, and CN0.95 (Supporting Information,
Figure S1 and Table S1). Typical TEM images demonstrate
that Pd NPs with an average size of 3.2 ꢁ 0.3 nm were
uniformly dispersed on the N-doped carbon supports (Sup-
porting Information, Figure S3). XPS results show that Pd had
a negative charge; this indicates a possible electron transfer
from CNx to Pd NPs (Supporting Information, Figure S4).
The effect of the CNx supports on the charge of the Pd NPs
was further investigated with ATR-IR spectroscopy,[9e] by
monitoring the adsorbed linear mode of CO on Pd/CNx
(Figure 1a). Nitrogen addition results in a lower wavenumber
than for the undoped counterpart; the extent of the red shift
was higher for samples with higher N content in the CNx
materials. These data confirm that CNx supports have a strong
electronic effect on Pd NPs, depending on the N content.
The catalytic properties of the Pd/CNx catalysts for FA
dehydrogenation were evaluated in a base-free aqueous
medium (1m FA) and in mild conditions (258C). For this
overall decarboxylation process, the adsorption of the FA
Figure 1. a) ATR-IR of CO on Pd/CNx catalysts. b) Correlation between
the surface pyridinic-N/Pd molar ratio and the initial TOF value of
Pd/CNx catalysts.
of 3625 h-1 per surface Pd site (Supporting Information,
Figure S5); the catalyst was selective, forming equivalent
amounts of H2 and CO2.
To verify and understand the effect of the surface
pyridinic-N, we modified commercial composite Pd-based
catalysts with N-containing model organic molecules (Sup-
porting Information, Table S3). Modification with pyrrole
decreased the performance, while pyridine and 3-amino-
pyridine improved the activity of both Pd catalysts. These
results confirm that pyridinic-N beneficially modulates the
electronic properties of the Pd catalyst surface, thereby
promoting H2 generation. Therefore, we prepared various
N-doped carbons (NDCs) with the general bio-chitosan-
based pyrolysis strategy; a family of N- and/or C-containing
chemicals, such as polyethyleneimine, urea, peptone, and
acetonitrile, were used. The N/C molar ratio of the formed
materials was maintained at 0.25 ꢁ 0.1 by changing the weight
of precursors used (Supporting Information, Table S4).[14]
Results in Figure S7 (Supporting Information) and images
in Figures 2a,b and Figure S8 (Supporting Information)
clearly show the mesoporous nature and the nanosheet-like
structure of these samples. For the supported catalysts, Pd
NPs with average sizes of 3.0 to 3.5 nm were uniformly
dispersed on supports. Notably, the sample that combined
chitosan and urea supported Pd NPs of 3.1 ꢁ 0.3 nm (Fig-
ure 2c) demonstrated a higher specific surface area of
465 m2 gꢀ1 (Supporting Information, Figure S7). HAADF-
ꢀ
molecules and the cleavage of the O H bond can be
promoted by modified surface Lewis base properties because
of the presence of the N dopant. The following key step of the
ꢀ
C H bond cleavage can be effectively chemically catalyzed
by the modulated Pd NPs on the surface of the electronic
structure.[9e] A plot of activity as a function of the N content of
the Pd/CNx catalysts did not show a linear correlation. In fact,
Pd/CN0.95 showed the strongest electronic effect between Pd
NPs and N-doped support but corresponded to the lowest
performance, while Pd/CN0.23 presented the highest TOF
(Supporting Information, Figure S5). To better understand
this phenomenon, we carried out further characterization.
Based on XPS analysis of the N 1s and of the surface metal
compositions, we found that there are three different N states
(graphitic-, pyrrolic-, and pyridinic-N),[11,12] as shown in
Figure S6 (Supporting Information). Pyridinic-N was shown
to be the main contributor to the electronic interaction with
Pd NPs, and the surface pyridinic-N/Pd molar ratio was
responsible for the catalytic activity (Figure 1b). This is in line
with the previous demonstrations of Pt-based catalysis.[13]
Remarkably, Pd/CN0.23 was found to be highly active for FA
decomposition, with an impressive turnover frequency (TOF)
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Pt, Ru, Rh, and Ir NPs supported on CN0.25 as reference
catalysts (Supporting Information, Figure S17) showed that
Pd is uniquely active on the N-doped carbon materials for
selective FA decomposition (Supporting Information,
Table S7). In contrast to the catalysts in which Pd with an
identical metal loading (10 wt%) is supported on conven-
tional metal oxides and reduced graphene oxide (rGO),[10b]
the Pd/CN0.25 hybrid exhibited the highest activity toward FA
decomposition into H2 (Supporting Information, Table S7).
Note that a previously reported catalyst of Pd/mpg-C3N4 with
a metal particle size of 3.0 ꢁ 0.2 nm (Supporting Information,
Figure S18)[15] showed a much lower activity (1247 hꢀ1).
The dependence of the FA concentration and reaction
temperature on the dehydrogenation rate of FA over a Pd/
CN0.25 was also studied. The rates of H2 release increased as
the FA concentration increased from 0.5 to 1.0m (Supporting
Information, Figure S19). However, higher values were found
to have a negative effect on dehydrogenation kinetics.
Despite the reduced activities at concentrations higher than
1.0m, the catalyst still showed enhanced activity compared to
previous results, even at concentration as high as 4.0m.[9d,e,j–l]
This represents a clear advantage for practical applications.
Moreover, 240 mL of gas can be readily generated within 1 h
over Pd/CN0.25 at 258C (Figure 3), corresponding to almost
full conversion (up to 97.5%) of FA into H2 and CO2. It is also
important to highlight that in all catalytic experiments,
ultraclean H2 gas was generated without the detection of
toxic CO, which is essential to enable direct downstream
applications in fuel-cell-based technologies for clean power
generation in a modern hydrogen economy.[5,10b,16] The
reaction rate was higher at higher temperatures, but even at
lower temperatures a significant H2 evolution (TOF up to
1449 hꢀ1 at 108C) can still be obtained (Figure 3). The
apparent activation energy (Ea) was estimated to be
48.8 kJmolꢀ1 (Supporting Information, Figure S21), which is
lower than that determined for the previously reported
heterogeneous catalysts.[9a,10a,17]
Figure 2. a) FESEM and b) TEM images of CN0.25 material. c) TEM and
d) HAADF-STEM images of Pd/CN0.25 catalyst.
STEM and TEM images (Figures 2d; Supporting Informa-
tion, Figure S9) showed the Pd NPs were distinctly anchored
on the underlying supports. Moreover, the surface of Pd NPs
obviously possessed a negative charge because of the
electronic interaction between metal and N-doped carbon
hybrids (Supporting Information, Figure S10).
These NDC-supported Pd catalysts were then tested for
FA dehydrogenation. To our delight, Pd/CN0.25 displayed the
highest ever reported activity of 5530 hꢀ1 at 258C (Supporting
Information, Table S4 and Figure S11).[9,10] For the supported
Pd catalysts with identical metal loading and similar particle
size, the lattice microstrains resulting from the lattice
mismatch between Pd and the underlying support are
negligible (Supporting Information, Figure S12).[10b] On the
basis of elemental analysis and XPS spectra of N 1s (Support-
ing Information, Table S5), an excellent and positive linear
correlation could be seen between the catalytic activity and
the surface pyridinic-N/Pd molar ratio (Supporting Informa-
tion, Figure S13); this is consistent with the results for
supported Pd systems made from chitosan and melamine
(Figure 1b). These data indicate that the Pd NPs can be
strongly modified with the electronic effect of NDCs—
especially by the surface pyridinic-N. This effect can induce
unique surface chemical properties in the Pd catalyst and lead
to high FA dehydrogenation performance.
Additionally, the heterogeneous nature and reusability of
this catalytic system was confirmed by the following results.
In the microenvironment of Pd NPs, an excess of
pyridinic-N may induce stronger adsorption of formate
intermediates; this can result in a decrease in the catalytic
activity (Supporting Information, Table S6). A Pd loading of
10 wt% on CN0.25 was beneficial to the metal–support
electronic interaction and surface metal atomic density, as
shown in Figure S16 (Supporting Information). Control
experiments with Pd-free CN0.25 show that the presence of
Pd was essential to achieve high FA dehydrogenation activity
(Supporting Information, Table S7). A comparison with Au,
Figure 3. H2 evolution by FA decomposition catalyzed by Pd/CN0.25
catalyst at different temperatures. Reaction conditions: aqueous FA
~
&
*
(1m, 5.0 mL scale), Pd (37.7 mmol); 108C ( ), 208C ( ), 258C ( ),
!
358C ( ).
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Pd/CN0.25 was removed from the reaction mixture at half FA
conversion. Further processing of the filtrate at 258C for 1 h
did not increase the conversion. Inductively coupled plasma
atomic emission spectroscopy (ICP-AES) analysis of the
filtrate showed that the content of Pd in the solution was
below the detection limit. These data confirmed that the
reaction is attributed to the heterogeneous catalysis of
Pd/CN0.25. This catalyst retains excellent stability for large
scale H2 release; an overall stream of 4.4 L of gas, corre-
sponding to a 90% conversion, can be readily attained from
100.0 mL of 1.0m FA within 600 min (Figure 4). As shown in
the inset to Figure 4, the catalyst was reused up to six times
without significant loss of catalytic efficiency; the 50040 total
turnover number (TON) reached the highest value yet
reported in clean heterogeneous systems, thus further
demonstrating the effectiveness of Pd/CN0.25 at room temper-
ature. Furthermore, the Pd/CN0.25 catalyst showed unique
robustness and was air stabile for half a year in ambient
conditions (Supporting Information, Figure S23).
incorporation of active Pd NPs on a matrix of flexible
pyridinic-N-doped carbon hybrids. By employing a controlled
synthetic approach involving a bio-chitosan-based pyrolysis
strategy, we have demonstrated that a further and comple-
mentary optimization of Pd NPs can be achieved by a simple
surface electronic modulation with the pyridinic-N group.
This method leads to an enhancement of FA dehydrogenation
catalytic efficiency as well as CO2 hydrogenation. The present
findings can make a significant contribution, not only in terms
of disclosing the electron-rich pyridinic-N-tuned Pd catalyst
as a means to boost catalytic efficiency, but also to establish
a viable and convenient H2 production process with a carbon-
neutral cycle for clean and sustainable energy storage.
Acknowledgements
This work was supported by NSF of China (21273044,
21473035, 91545108, and 51502331), Science & Technology
Commission of Shanghai Municipality (16ZR1440400), and
the Open project of State Key Laboratory of Chemical
Engineering (SKL-ChE-15C02).
Keywords: formic acid · heterogeneous catalysis ·
hydrogen storage · Pd nanoparticles · pyridinic-N-doped carbon
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Received: June 20, 2016
Revised: July 18, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Hydrogen Storage Materials
Q. Y. Bi, J. D. Lin, Y. M. Liu, H. Y. He,
Pyridinic-N-tuned catalysis: An electron-
rich pyridinic-N dopant modulates the
electronic interactions between the active
sites of palladium nanoparticles and the
carbon support. Formic acid dehydro-
genation at room temperature is signifi-
cantly boosted by the pyridinic-N-doped
palladium catalyst, presenting an efficient
and reliable route to clean H2 generation
and sustainable energy storage.
F. Q. Huang, Y. Cao*
&&&& — &&&&
Dehydrogenation of Formic Acid at Room
Temperature: Boosting Palladium
Nanoparticle Efficiency by Coupling with
Pyridinic-Nitrogen-Doped Carbon
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!