Sodium hydroxide-assisted growth of uniform Pd nanoparticles on nanoporous carbon MSC-30 for efficient and complete dehydrogenation of formic acid under ambient conditions

Article abstract of DOI:10.1039/c3sc52448e

Highly dispersed Pd nanoparticles (NPs) deposited on nanoporous carbon MSC-30 have been successfully prepared with a sodium hydroxide-assisted reduction approach. The modification by NaOH during the formation and growth of particles results in the well-dispersed ultrafine Pd NPs on carbon. The combination of distinct interaction between metal and support and high dispersion of NPs drastically enhances the catalytic performance of the resulted catalyst, over which the turnover frequency (TOF) for heterogeneously catalyzed decomposition of formic acid (FA) reaches 2623 h^-1 at 50 °C with 100% H₂ selectivity, the highest value ever reported under ambient conditions, comparable to those acquired from the most active homogeneous catalysts. Even at 25 °C, the complete dehydrogenation of FA with a TOF as high as 750 h^-1 can be achieved.

Full text of DOI:10.1039/c3sc52448e

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Sodium hydroxide-assisted growth of uniform Pd
nanoparticles on nanoporous carbon MSC-30 for
efficient and complete dehydrogenation of formic
acid under ambient conditions†
Cite this: Chem. Sci., 2014, 5, 195
a
Qi-Long Zhu,a Nobuko Tsumoriab and Qiang Xu
*
Highly dispersed Pd nanoparticles (NPs) deposited on nanoporous carbon MSC-30 have been successfully
prepared with a sodium hydroxide-assisted reduction approach. The modification by NaOH during the
formation and growth of particles results in the well-dispersed ultrafine Pd NPs on carbon. The
combination of distinct interaction between metal and support and high dispersion of NPs drastically
enhances the catalytic performance of the resulted catalyst, over which the turnover frequency (TOF) for
heterogeneously catalyzed decomposition of formic acid (FA) reaches 2623 hꢀ1 at 50 ꢁC with 100% H2
selectivity, the highest value ever reported under ambient conditions, comparable to those acquired
from the most active homogeneous catalysts. Even at 25 ꢁC, the complete dehydrogenation of FA with a
TOF as high as 750 hꢀ1 can be achieved.
Received 31st August 2013
Accepted 15th October 2013
DOI: 10.1039/c3sc52448e
www.rsc.org/chemicalscience
endowing itself with an important advantage over other
hydrogen carriers, hence preventing the accumulation of by-
Introduction
products which is a limitation for portable use.
Among the various alternative energy strategies, hydrogen, an
environmentally attractive energy carrier that has been consid-
ered one of the ultimate energy vectors to connect a host of
energy sources to diverse end users for “mobile applications”,
will enable a secure and clean energy future.1 Controlled storage
and release of hydrogen in a safe and efficient way remains one
of the most difficult challenges toward the fuel cell based
hydrogen economy.2 Recently, formic acid, a product of
biomass processing and photocatalytic CO2 reduction with high
energy density, non-toxicity and excellent stability, has been
identied as a safe and convenient hydrogen carrier.3–7 Notably,
formic acid as liquid hydrogen fuel possesses the distinguished
advantages of easy recharging and the availability of the current
liquid fuel infrastructure for recharging. Hydrogen stored in FA
can be released via a catalytic dehydrogenation reaction
(HCOOH / H2 + CO2, DG298K ¼ ꢀ48.8 kJ molꢀ1).3b However,
the undesirable dehydration pathway (HCOOH / H2O + CO,
DG298K ¼ ꢀ28.5 kJ molꢀ1), which is typically promoted by
heating or acidity, should be strictly controlled, as CO impuri-
ties are not well tolerated by fuel cells.3 Only gaseous products
(H2/CO2) are formed from selective decomposition of FA,
Although the decomposition of formic acid has been
intensely investigated for hydrogen generation, catalysts pos-
sessing prominent activity with facile and selective evolution of
CO-free H2 gas under ambient conditions are still few.3
Recently, noteworthy advances have been made in the eld of
FA dehydrogenation with different homogeneous catalysts.4,5
Beller and co-workers developed a series of Ru–phosphine
complexes,5 with which an excellent performance for FA
decomposition with an initial turnover frequency (TOF) of 3630
hꢀ1 from FA–amine mixtures at 40 ꢁC has been obtained.5a
Laurenczy and co-workers reported a specically designed
water-soluble Ru–TPPTS system, which can release a pressur-
ized H2–CO2 mixture from aqueous solution of FA–sodium
formate (SF) at temperatures of 70–120 ꢁC.4a,b More recently,
Laurenczy, Beller and co-workers reported a highly active iron
ꢁ
catalyst system for the liberation of H2 from FA at 80 C in a
solution of FA in environmentally benign propylene carbo-
nate.4h In contrast to the great progress achieved in the FA
dehydrogenation with homogeneous systems, the heteroge-
neously catalyzed dehydrogenation of FA at ambient tempera-
ture is not yet well developed.6 Cao and co-workers applied an
ultra-dispersed gold catalyst comprising of gold sub-
nanoclusters deposited on zirconia to a FA–amine mixture,
aNational Institute of Advanced Industrial Science and Technology (AIST), Ikeda,
Osaka, 563-8577, Japan. E-mail: q.xu@aist.go.jp; Fax: +81-72-751-9629; Tel: +81-
72-751-9562
affording a TOF value as high as 1590 hꢀ1 6e Recently, supported
.
bToyama National College of Technology, 13, Hongo-machi, Toyama, 939-8630, Japan
Pd-based nanocatalysts have been found to be active for the
decomposition of FA, while their catalytic activities and selec-
tivities are poor even at elevated temperature up to 90 ꢁC.6b,7 The
† Electronic supplementary information (ESI) available: Experimental procedures;
GC, PXRD, BET, XPS, TEM for catalysts; results of catalytic dehydrogenation of FA;
durability and stability results of catalysts. See DOI: 10.1039/c3sc52448e
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development of highly active and selective heterogeneous evaluated on the basis of the volumetrically measured amount
catalysts for the decomposition of FA in the liquid phase under of gas released during the reaction.
mild conditions is desired urgently for practical use.
Remarkably, the Pd/MSC-30 prepared in the presence of base
In light of the previous work that the interaction between exhibits complete and selective decomposition of FA into H2
metal and support and high dispersion of metal NPs may be and CO2 with an exceedingly high activity. 450 mL of gas
conducive to the catalytic performance of nanocatalysts, the without CO detected can be generated within 2.33 min in a FA–
ꢁ
type of support and the method for preparing ultrane metal SF system at 50 C (Fig. 1a), which is composed of 440 mL of
NPs are important for promoting the kinetic properties of FA CO2 + H2 generated from complete FA decomposition and
dehydrogenation.6d,e Herein, we report complete and efficient 10 mL of H2 contributed by the hydrolysis of SF (Fig. 1c), indi-
H2 generation without CO contamination from FA in an cating complete and efficient conversion of FA into H2 and CO2.
aqueous FA–SF solution at ambient temperature, catalyzed by a The exclusive formation of H2 and CO2 without CO formation
nanoporous carbon supported Pd nanocatalyst which is has been conrmed by gas chromatography (GC) analyses
prepared by a facile sodium hydroxide-assisted reduction (Fig. S1 and S2†), which is crucial for fuel cell applications.9
method. Surprisingly, the resultant catalyst affords a TOF value Particularly noteworthy is that the Pd catalyst gave an excep-
as high as 2623 hꢀ1 at 50 ꢁC with 100% H2 selectivity for tional turnover frequency (TOF) of 2623 hꢀ1, the highest activity
heterogeneously catalyzed decomposition of formic acid, the ever reported for heterogeneously catalyzed FA decomposition,
highest value ever reported under ambient conditions, compa- even comparable to the most active homogeneous catalysts
rable to those acquired from the most active homogeneous (Table S1†).3–7,10 The average rate of H2 production was deter-
catalysts. Maxsorb MSC-30, an elaborate nanoporous carbon, mined to be 684 L H2 hꢀ1 gPdꢀ1, cꢀo1rresponding to a theoretical
was selected as the support not only because of its incredibly power density of 923 W hꢀ1 gPd for energy generation.6c,e,f
large pore size (2.1 nm) and high specic surface area (>3000 m2 Accordingly, taking an operation efficient of 60% and a typical
gꢀ1),8 but also because of the possible enhanced interaction energy requirement value of 0.5–2.0 W h for portable terminals,
between the metal and carbon support. The key to obtaining the one gram of the present Pd/MSC-30 catalyst would be sufficient
excellent catalytic properties of Pd/MSC-30 is the ligand to supply H2 for 27–110 small proton exchange membrane
exchange of the precursor complex prior to the reduction with (PEM) fuel cell devices.6e,11 To be emphasized, few solid catalysts
NaBH4, which results in the well-dispersed ultrane Pd NPs. have been reported to be highly active for the decomposition of
Moreover, SF in the reaction system plays an important role as a FA with high selectivity at low temperature.6e Most reported
catalyst promoter.
nanocatalysts for FA decomposition usually lose their initially
good activities within a short time at the elevated reaction
temperature, which may be due to the fact that at elevated
temperatures, CO is easier to be evolved to poison the catalysts,
and the stability of the catalysts in their size and structure is
decreased. To our delight, even at a temperature as low as 25 ꢁC,
complete FA decomposition can be readily achieved with TOF of
750 hꢀ1 (Fig. 2). The molar ratio of FA to SF has an obvious effect
on the performance of the resulted Pd/MSC-30 catalyst. The
Results and discussion
A facile sodium hydroxide-assisted reduction approach was
adopted for the preparation of the Pd/MSC-30 nanocatalyst
(Scheme 1). Briey, 100 mg of MSC-30 carbon was suspended in
2.50 mL water, to which 0.30 mL of aqueous K2PdCl4 solution
(0.30 M) was added. The resulting aqueous suspension was
subsequently sonicated to homogenize the content, and then
treated with 0.50 mL of NaOH solution (2 M). Subsequent
reduction using NaBH4 under vigorous stirring generated the
product of Pd/MSC-30, which served as an efficient catalyst for
H2 generation from FA. Catalytic dehydrogenation of FA was
initiated by the introduction of aqueous FA–SF solution into the
reaction ask containing Pd/MSC-30 catalyst with vigorous
shaking. The catalytic performance of the catalysts was
Fig. 1 Volume of the generated gas (CO2 + H2) versus time for the
dehydrogenation of FA–SF (1 : 1) over Pd/MSC-30 prepared (a) with
and (b) without 0.5 mL of 2.0 M NaOH added (nPd/nFA ¼ 0.01, 50 ꢁC),
and (c) the hydrolytic dehydrogenation of SF over Pd/MSC-30
prepared with NaOH (nPd/nSF ¼ 0.01, 50 ꢁC). Inset: TOF values of H2
Scheme 1 Schematic illustration for Pd/MSC-30 nanocatalyst prep- generation from FA–SF over Pd/MSC-30 prepared with different
aration via a sodium hydroxide-assisted reduction approach and gas amounts of 2.0 M NaOH added to 2.8 mL precursor solution (K2PdCl4,
(CO2 + H2) evolution from the dehydrogenation of FA–SF.
0.09 mmol).
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prepared with 0.5 mL of 2.0 M NaOH exhibited the best catalytic
performance in the dehydrogenation of FA (inset in Fig. 1 and
S8†), suggesting that a small amount of base during NPs growth
is sufficient to improve the catalytic activity of Pd/MSC-30.
As a unique carbon with nanoporous nature, MSC-30 is
superior to other kinds of carbon as a support for catalysts. For
comparison, Pd NPs supported on another porous carbon,
Vulcan XC-72R, with a surface area of 240 m2 gꢀ1 was synthe-
sized in the presence of NaOH. Surprisingly, despite their
similar particle size, distribution and crystal structure (vide
infra), the catalytic activity of Pd NPs grown on XC-72R is much
lower than those on MSC-30 for FA decomposition (Fig. S11†),
highlighting the strong interaction between the active NPs and
MSC-30. To further determine the effect of the choice of the
underlying support on the catalytic performance, we also eval-
uated Pd NPs supported on Al2O3 nanoparticles (Pd/Al2O3),
which showed signicantly lower activity under the same
condition (Fig. S11†). Based on the above results, it is reliable to
believe that the nature of MSC-30 as a support is capable of
providing a specic interaction with Pd NPs, which may be the
key factor for facilitating the dehydrogenation of FA.
Fig. 2 (a) Volume of the generated gas (CO2 + H2) versus time and (b)
corresponding TOF values of H2 generation for the dehydrogenation
of FA–SF (1 : 1) at different temperatures over Pd/MSC-30 prepared
with NaOH (nPd/nFA ¼ 0.01). Inset of (a): Arrhenius plot (ln(TOF) vs. 1/T).
A full characterization of the Pd/MSC-30 catalysts was per-
formed. The transmission electron microscopy (TEM) and high-
angle annular dark-eld scanning TEM (HAADF-STEM) images
of the Pd/MSC-30 prepared with NaOH show that the Pd NPs are
highly dispersed into the framework of MSC-30 with an average
activity of the catalyst for decomposition of FA increases with particle size of 2.3 ꢂ 0.4 nm based on TEM observations, despite
the molar percentage of SF in aqueous FA–SF solution until the very few aggregated NPs being observed (Fig. 3a, b and S6a†).
value reaches 50%, aer which further increase in the The corresponding selected area energy dispersion (SAED)
percentage of SF has almost no effects on the decomposition of pattern indicates the crystalline nature of the Pd NPs (inset in
FA (Fig. S9†). In addition, the hydrogen evolution rate greatly Fig. 3a). In addition, appreciable decreases in the amount of N2
depends on the reaction temperature (Fig. 2). The apparent adsorption are observed aer the deposition of Pd NPs
activation energy (Ea) of this process was estimated to be 38.6 kJ (Fig. S3†), indicating that the pores of the host framework of
molꢀ1, which is lower than most of those reported for both MSC-30 are occupied by dispersed Pd NPs and/or blocked by the
heterogeneous and homogeneous catalyst systems for FA NPs located on surface as in the case of metal NPs loaded onto
dehydrogenation.4–6 Thus, FA can be efficiently converted into a
CO-free H2-containing stream at convenient temperature by Pd/
MSC-30 prepared in an alkaline solution.
In addition, the stability of the Pd/MSC-30 catalyst was
examined by successively adding aliquots of pure FA (0.34 mL,
9.0 mmol) into the reactor aer the completion of previous run,
and no signicant loss in activity and selectivity was observed
over 5 cycles (Fig. S10†). Aer the catalytic reaction, Pd/MSC-30
shows a similar crystal structure and particle size distribution
as that before the reaction (Fig. S14 and S15†). The results
reliably indicate that the present catalyst possesses high dura-
bility and stability during FA decomposition.
With the absence of NaOH during catalyst preparation,
Pd/MSC-30 exhibits a signicant decrease in catalytic activity
for the decomposition of FA (Fig. 1b), although its apparent
activation energy was calculated to be 39.1 kJ molꢀ1, which is
similar to that for Pd/MSC-30 prepared with NaOH (Fig. S12†). It
is concluded that the method of catalyst preparation is
extremely important for the improvement of kinetics. Further,
we have prepared Pd/MSC-30 with various amounts of 2.0 M
NaOH solution (0.5 to 2.0 mL) added in 2.8 mL precursor
Fig. 3 (a) TEM (inset SAED) and (b) HAADF-STEM images of Pd/MSC-
30 prepared with NaOH, (c) TEM image of Pd/MSC-30 prepared
without NaOH, and (d) EDX pattern of the selected area in (b). The
solution (K2PdCl4, 0.09 mmol) and found that the Pd/MSC-30 copper signals originate from the TEM grid.
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porous metal–organic frameworks (MOFs).12 However, when Pd aer a long treatment with alkaline solution. Thus, the forma-
NPs were loaded without NaOH, a larger average particle size of tion of colloidal Pd(OH)2 can be excluded during the formation
3.6 ꢂ 0.6 nm was observed (Fig. 3c and S6b†), although the X-ray of the Pd NPs on carbon in the presence of NaOH. Conse-
diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) quently, such ligand exchange of Clꢀ for OHꢀ prior to the
analyses show that the two catalysts possess similar crystalline reduction with NaBH4 accounts for the decrease in particle size
and electronic structures (Fig. S4 and S5†). Therefore, NaOH of Pd NPs, which is probably due to the enhanced electronic
serves as an efficient dispersing agent to control the size interaction between the chlorohydroxypalladium(II) species and
distribution during the formation of Pd NPs in the present carbon support. Namely, the precursor complexes containing
system. It has been known that the catalytic activity generally OHꢀ ligands are much more favorable for the formation of
increases with decreasing metal NP size, as smaller particles small Pd NPs on the supports than those without OHꢀ ligands.
possess higher surface areas available for reactants.6g,13 Since
there is no signicant difference in the crystalline and elec-
tronic properties of Pd particles between the two nanocatalysts,
Conclusions
it is reasonable to believe that the decrease in particle size of Pd
In summary, a facile strategy has been successfully applied by
NPs on MSC-30 due to the presence of NaOH should be
using a sodium hydroxide-assisted reduction approach for the
responsible for the drastic enhancement of the catalytic
preparation of well-dispersed ultrane Pd NPs, which is
induced by the ligand exchange of the precursor complex prior
to the reduction with NaBH4. The nanoporous carbon MSC-30
proved to be a distinct and powerful support for the Pd-based
catalyst with strong interaction between metal and support.
Catalyzed by the resultant Pd/MSC-30, a highly efficient and
complete hydrogen generation from FA in a FA–SF system
performance. The highly dispersed Pd NPs on XC-72R and Al2O3
supports with average particle sizes of 2.8 ꢂ 0.5 and 3.3 ꢂ
0.3 nm, respectively (Fig. S7†), further demonstrate that NaOH
plays an important role in controlling the growth of small-
size Pd NPs.
In order to investigate the reason for the change in particle
size of Pd NPs induced by NaOH addition, we tried to identify
without undesired CO contamination, and with the highest TOF
the coordination structure of the precursor in the absence and
value in the heterogeneously catalyzed FA dehydrogenation,
presence of NaOH by using UV-Vis absorption spectroscopy.
even comparable to those acquired from the most active
The metal precursor in aqueous solution existed as a hydrated
homogeneous catalysts, was achieved under mild conditions.
ion [PdCl3(H2O)]ꢀ, which was conrmed by its ligand-to-metal
This readily accessible and highly effective monometallic cata-
charge transfer (LMCT) band around 235 nm (Fig. 4).14 Aer
lyst is believed to strongly promote the practical development of
adding an amount of NaOH corresponding to ten equivalents of
formic acid as a viable hydrogen storage medium for fuel cells
K2PdCl4 to the precursor aqueous solution, the LMCT band of
and chemical synthesis applications.
[PdCl3(H2O)]ꢀ complex disappeared immediately, along
with the appearance of a broad absorption band centered at
268 nm, which can be assigned to the LMCT band of mixed Acknowledgements
chlorohydroxypalladium(II) species, e.g., [PdCl3(OH)]2ꢀ and
The authors are thankful to the reviewers for valuable sugges-
tions, Dr Takeyuki Uchida for TEM measurements and AIST and
METI for nancial support.
[PdCl2(OH)2]2ꢀ
. However, the intensity of the new absorption
15
band was maintained by further addition of aliquots of NaOH,
indicating complete ligand exchange (Fig. S13†). Moreover, a
noticeable decrease of the 268 nm band can only be observed in
the spectra, along with the observation of some precipitation,
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