Ultrasmall palladium nanoparticles supported on amine-functionalized SBA-15 efficiently catalyze hydrogen evolution from formic acid

Article abstract of DOI:10.1039/c4ta04538f

The success of the so-called hydrogen economy for large-scale applications will ultimately depend on efficient and sustainable production, storage and distribution of hydrogen. Owing to its low toxicity, high volumetric H₂ storage capacity and availability both from renewable resources (e.g., biomass) and non-renewable resources (e.g., fossil fuel feedstocks), formic acid is one of the most favorable chemical hydrogen storage media for large-scale energy storage applications. However, for FA to become a viable hydrogen storage medium, efficient catalysts that enable it to release H₂ at low cost are necessary. Herein we report a facile synthetic route to amine-functionalized nanoporous silica-supported ultrasmall Pd nanoparticles (SBA-15-Amine/Pd) that are highly active catalysts for formic acid dehydrogenation, producing hydrogen at ambient temperature with a high turn-over-frequency (TOF) of 293 h^-1, which is among the highest TOFs ever reported for the reaction by a heterogeneous catalyst. We also show that the material is easily recyclable multiple times, without losing its catalytic activity. So, the catalyst we developed can be expected to be part of the solutions of our sustainability challenges.

Full text of DOI:10.1039/c4ta04538f

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Materials Chemistry A
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Ultrasmall palladium nanoparticles supported on
amine-functionalized SBA-15 efficiently catalyze
hydrogen evolution from formic acid†
Cite this: J. Mater. Chem. A, 2014, 2,
20444
a
d
d
ab
de
abc
Received 1st September 2014
Accepted 15th October 2014
*
*
*
and T. Asefa
K. Koh, J.-E. Seo, J. H. Lee, A. Goswami,
C. W. Yoon
DOI: 10.1039/c4ta04538f
www.rsc.org/MaterialsA
The success of the so-called “hydrogen economy” for large-scale of the most promising clean energy carriers—can play pivotal
applications will ultimately depend on efficient and sustainable roles in meeting these challenging issues, as already success-
production, storage and distribution of hydrogen. Owing to its low fully demonstrated with clean hydrogen-fed fuel cell technolo-
toxicity, high volumetric H2 storage capacity and availability both from gies.1–4 Despite the progress made since the 1970s, when the
renewable resources (e.g., biomass) and non-renewable resources term “hydrogen economy” was coined by Bockris,2 it has
(e.g., fossil fuel feedstocks), formic acid is one of the most favorable become apparent that the ultimate success of the hydrogen-
chemical hydrogen storage media for large-scale energy storage based energy cycle for large scale applications heavily hinges on
applications. However, for FA to become a viable hydrogen storage efficient, large-scale production, storage, and distribution of
medium, efficient catalysts that enable it to release H2 at low cost are hydrogen. In particular, the development of safe and reversible
necessary. Herein we report a facile synthetic route to amine-func- hydrogen storage systems is a key technical issue that must be
tionalized nanoporous silica-supported ultrasmall Pd nanoparticles fully addressed to successfully realize an “ideal” hydrogen
(SBA-15-Amine/Pd) that are highly active catalysts for formic acid economy.5–7
dehydrogenation, producing hydrogen at ambient temperature with a
Because of its low toxicity, high hydrogen storage capacity and
high turn-over-frequency (TOF) of 293 hÀ1, which is among the availability both from renewable resources (e.g., biomass) and non-
highest TOFs ever reported for the reaction by a heterogeneous renewable resources (e.g., fossil-feedstock), formic acid (FA) has
catalyst. We also show that the material is easily recyclable multiple recently attracted signicant attention for its potential applications
times, without losing its catalytic activity. So, the catalyst we devel- as a chemical hydrogen storage medium.8,9 FA's high volumetric
oped can be expected to be part of the solutions of our sustainability H2 storage capacity of 53 g LÀ1 particularly makes it advantageous
challenges.
for many on-board energy applications.10 Moreover, in presence of
suitable catalysts, FA can be dehydrogenated to produce the
hydrogen it carries under ambient conditions, making it auspi-
cious for direct utilization within hydrogen-fed polymer electrolyte
membrane-based fuel cells (PEMFCs).
Given the current energy landscape, where the consumption of
fossil fuels continues to rise while the negative environmental
impacts associated with burning fossil fuels continue unabated,
the search for alternative, benign and sustainable energy sour-
ces is of burgeoning interest. In this context, hydrogen—as one
For FA to be used as a viable hydrogen storage medium
though, efficient catalysts that enable it to generate H2 at low
cost are critically needed. In recent years, various homogeneous
catalysts based on metals such as Ru 11–14 and Ir 15 and hetero-
geneous catalysts including AuPd/C,16 AgPd,17 CoAuPd/C,18 Pd/
aDepartment of Chemistry and Chemical Biology, Rutgers, The State University of New
Jersey, 610 Taylor Road, Piscataway, New Jersey 08854, USA. E-mail: tasefa@rci.
rutgers.edu; agoswami@rci.rutgers.edu
19,20
C3N4
for the dehydrogenation of FA have been reported.
Generally, homogeneous catalysts are highly active and exhibit
higher turnover frequencies (TOFs) than heterogeneous cata-
lysts for many reactions including formic acid dehydrogena-
tion. However, the heterogeneous catalysts are advantageous
over homogeneous ones in terms of recoverability of the spent-
catalysts from the reaction mixtures.21 By immobilizing homo-
geneous catalysts onto solid support materials, the benets
attainable by both types of catalysts can sometimes be realized.
This approach of ‘heterogenization of homogeneous catalysts’
bDepartment of Chemical and Biochemical Engineering, Rutgers, The State University
of New Jersey, 98 Brett Road, Piscataway, New Jersey 08854, USA
cInstitute for Advanced Materials, Devices and Nanotechnology (IAMDN), Rutgers, The
State University of New Jersey, 607 Taylor Road, Piscataway, New Jersey 08854, USA
dFuel Cell Research Center, Korea Institute of Science and Technology, Hwarangno14-
gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea. E-mail: cwyoon@kist.re.kr
eDepartment of Clean Energy and Chemical Engineering, University of Science and
Technology, Daejeon, Republic of Korea
† Electronic supplementary information (ESI) available: Detailed experimental
procedures, TEM, FT-IR, and TOF calculation data. See DOI: 10.1039/c4ta04538f
20444 | J. Mater. Chem. A, 2014, 2, 20444–20449
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can also be successfully applied for catalytically active ultra- otherwise identical synthetic conditions afforded bigger Pd
small metallic nanoclusters, which have increasingly become NPs (Fig. 1c and d). The latter is not surprising since ultra-
promising catalysts for formic acid dehydrogenation.22–26 small Pd NPs are oen too difficult to obtain with conven-
Furthermore, functional groups such as amines could easily be tional reduction methods employing NaBH4 and/or without
supported on the surfaces of such heterogeneous catalysts to capping agents because they tend to aggregate easily due to
serve as interaction sites for supported metallic nanoclusters as their relatively high surface energies. In another control
well as co-catalytic species, which in the case of FA-dehydroge- experiment, the Pd NPs formed following the reduction of the
nation can function as deprotonation sites for FA molecules, ammoniacal Pd(II) species in the absence of SBA-15-Amine
synergistically assisting FA's catalytic dehydrogenation. This were also, unsurprisingly, signicantly bigger in size, having
hypothesis, in conjunction with the recent successful demon- particle sizes as high as 150 nm (Fig. S2†). Thus, the presence
stration of silica nanosphere-encapsulated Au nanoparticles26 of amine functional groups inside the mesoporous silica
and silica microsphere22 and metal–organic framework (MOF)24 channels and their ability to stabilize the Pd NPs must have
supported Pd catalysts for FA dehydrogenation, has prompted played important roles in keeping the Pd NPs quite small
us to explore other related but different systems. This has ulti- during the reduction of Pd2+ ions under H2 at 250 ꢀC. The
mately led us to synthesize hybrid catalytic systems composed amounts of Pd in the SBA-15-Amine/Pd and SBA-15/Pd, as
of ultrasmall Pd nanoparticles (Pd NPs)/amine-functionalized characteriÀz1ed by ICP-AES, were found to be identical (0.32
mesoporous silica, as shown in Scheme 1, that can efficiently mmol gcat , 3.4 wt%). On the basis of a previous report,31 it is
catalyze FA dehydrogenation.
conceivable that the amine functional groups graed in our
First, SBA-15 type mesoporous silica was synthesized SBA-15-Amine/Pd could serve as Brønsted basic sites, facili-
using a supramolecular self-assembly synthetic method, as tating the deprotonation of FA into a formate intermediate,
demonstrated by Zhao et al.27 By using the residual surface while the ultrasmall Pd NPs around them catalytically acti-
silanols (^Si–OH) located on the pore walls of SBA-15, the vate the C–H bond of the resulting formate species. Besides
material was functionalized with organic functional groups, this, the amine groups help with the formation as well as the
which were then utilized to anchor metal ions onto the stabilization of the ultrasmall Pd NPs (by protecting them
surfaces of the nanoporous material.28,29 In this current from aggregation during the catalytic reactions (see below)).
effort, primary amine groups were anchored on SBA-15 by
Furthermore, the amine groups supported onto SBA-15
graing (3-aminopropyl)triethoxysilane (APTES) onto the can donate electron density to Pd, thereby enhancing Pd NPs’
surfaces of SBA-15, producing a material denoted as SBA-15- catalytic activity towards FA dehydrogenation.31 To elucidate
Amine (Scheme 1). Then, Pd2+ ions were supported onto the this possible electronic effect of the amine groups on Pd, X-
surfaces of SBA-15-Amine by mixing the latter with ammo- ray photoelectron spectroscopy (XPS) was employed to
niacal palladium solution (Pd(NH3)4Cl2).30 Subsequent analyze the materials, and the results are displayed in Fig. 2,
reduction of the resulting material using a H2/N2 mixture S2 and S3.† As seen in the spectra, while the peaks corre-
ꢀ
(1 : 9 v/v) at 250 C for 3 h afforded SBA-15-Amine-supported sponding to Pd(0) in SBA-15/Pd with maxima are centered at
Pd NPs (denoted hereaer as SBA-15-Amine/Pd) that are 341.0 eV (3d3/2 state) and 335.8 eV (3d5/2 state),32 the corre-
highly catalytically active for FA dehydrogenation (see sponding peaks in the case of SBA-15-Amine/Pd are shied to
detailed experiments in the ESI†).
lower binding energies of 340.1 eV and 335.0 eV, respectively.
The structure and morphology of SBA-15-Amine/Pd were This is likely due to the interaction between Pd NPs and
rst analyzed by high-resolution transmission electron amine groups in SBA-15-Amine/Pd; i.e., the amine groups
microscopy (HRTEM) and high-angle annular dark-eld donate some electron density to Pd NPs and produce elec-
scanning transmission electron microscopy (HAADF-STEM). tron-rich Pd centers with lower binding energies. These
The Pd nanoparticles in SBA-15-Amine/Pd were found to be electronically perturbed Pd centers in SBA-15-Amine/Pd
ultrasmall, having an average size of 1.9 nm (Fig. 1a and b). In could thus be responsible for the material's improved cata-
contrast, the unmodied SBA-15/Pd prepared under lytic activity towards FA dehydrogenation compared with that
Scheme 1 A synthetic scheme showing the preparation of ultrasmall Pd nanoparticles supported on amine-functionalized SBA-15 mesoporous
silica, which can efficiently catalyze formic acid (FA) dehydrogenation.
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Fig. 1 TEM (a) and STEM (b) images of SBA-15-Amine/Pd. TEM (c) and STEM (d) images of SBA-15/Pd. The insets show the size distributions of the
particles obtained from the TEM images.
external bases using the set-up shown in Fig. S4.† As illustrated
in Fig. 3, among these various materials we investigated, SBA-
15-Amine/Pd showed superior activity towards FA dehydroge-
nation. The rate and amount of total H2-release from FA over
SBA-15-Amine/Pd were quite good, with an initial TOF of 293
hÀ1 in 10 min, which is among the highest TOFs for FA dehy-
drogenation previously reported for other heterogeneous cata-
lysts under ambient conditions without bases.11–20 The TOFs of
recently developed heterogeneous catalysts without additives at
ambient temperature are compared in Table S1† entries 1–6 and
Fig. S5.† It is also valuable to compare with a recently reported
most active palladium catalyst, Pd/MSC-30, under ambient
conditions that employed sodium formate along with formic
acid in the reaction (Table S1,† entry 7).33 The control materials,
SBA-15/Pd and Pd NPs, showed only little catalytic activity, again
Fig. 2 The high resolution Pd3d peaks in the XPS spectra of SBA-15-
Amine/Pd and SBA-15/Pd.
supporting that the amine functional groups in SBA-15-Amine/
Pd play a pivotal role in accelerating FA dehydrogenation. It is
also worth noting that the mesoporous silica material has no
activity on its own towards the reaction.
of SBA-15/Pd (vide infra). It is worth adding here that the Pd3d
To further scrutinize the potential roles of the covalently
spectra of both materials showed two small peaks corre-
tethered amine functional groups in the catalytic activity of the
sponding to Pd2+ centered at 343.2 eV and 337.9 eV, respec-
material towards FA dehydrogenation, we carried out additional
tively, indicating the negligible presence of Pd2+ species.
Next, we evaluated the catalytic activities of SBA-15-Amine/
Pd, SBA-15-Amine, SBA-15/Pd, and bare Pd NPs toward FA
controlled experiments by using the physical mixtures of SBA-
15/Pd and various quantities of n-propylamine (PA) as catalysts.
Compared with SBA-15/Pd, the physical mixture of SBA-15/Pd
dehydrogenation at room temperature in the absence of
and 0.1 mmol of PA showed an increased catalytic activity
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because the second (dehydration) process does not produce H2,
not to mention it forms CO – a common poison for many
catalysts, including the Pt catalysts typically used in hydrogen-
fed PEMFCs.35–39 To examine the selectivity of the overall reac-
tion to the one leading to the desired product, i.e., H2, dened
as [moles of H2/moles of (H2 + CO)] Â 100, FT-IR spectroscopy
was employed to detect CO directly (Fig. S6†). The FT-IR spectra
obtained using SBA-15-Amine/Pd aer 50 min gave a doublet
peak located at ca. 2300 cmÀ1, indicating the evolution of CO2.
The peak at ca. 2100 cmÀ1 corresponding to CO was barely
observable, strongly suggesting that the H2 selectivity is close to
ca. 100%.
Recyclability studies of the catalytic material were addi-
tionally carried out for SBA-15-Amine/Pd. The result revealed
that the catalyst was reusable at least six times without losing
its activity (Fig. 4). Interestingly, the TOFs were, in fact, found
to increase aer the 1st run and become nearly identical for
2nd to 6th runs, reaching values as high as 377 hÀ1
.
Consistent with these results, the TEM images of SBA-15-
Amine/Pd revealed no appreciable change in the structure of
mesoporous silica even following completion of the 6th
catalytic recycle run. Moreover, the size of Pd NPs also
remained unchanged (Fig. 5). The improved performance of
the recycled catalysts particularly aer the rst cycle can be
attributed to the reductive environment (due to the H2
produced by the reaction) that the catalyst is exposed to
during the catalytic reactions, which can convert any surface-
oxidized PdO (possibly present on the original catalyst) back
to the catalytically active Pd NPs. The reduction condition
can also prevent the Pd NPs from undergoing oxidation and
losing their activity. Finally, to determine possible leaching
of Pd from SBA-15-Amine/Pd during the reactions, the
supernatant obtained aer 150 min was examined by ICP-
AES. The amount of Pd in the supernatant was found to be
only ca. 10 ppm, indicating that the Pd NPs remain supported
on SBA-15-Amine/Pd during the dehydrogenation reaction.
Fig.
3 FA dehydrogenation catalyzed by the various materials
synthesized and investigated here. (a) The volume of total gases
produced versus time (min) curves. (b) TOFs at 10 min calculated based
on the mol of Pd in the materials (catalysts). Reaction conditions: 10
mL of 1 M FA was stirred with 50 mg of a desired catalyst (16 mmol of
palladium) at room temperature at 300 rpm.
(Fig. 3a and b). Note that the amount of amine or PA added is
0.1 mmol, which is almost equal to that of covalently tethered
amines in SBA-15-Amine/Pd. Upon addition of a higher amount
(5 mmol) of PA (Fig. 3a, indicated with pink triangles), the
corresponding activity increased slightly, but was still signi-
cantly lower than that obtained with SBA-15-Amine/Pd. Never-
theless, these results strongly imply that the added amine
clearly acted as a Brønsted base, helping the deprotonation of
FA. The results also suggest that the amine functional groups on
the pore walls of SBA-15-Amine/Pd play multiple functions; i.e.,
effectively deprotonating FA, besides facilitating the synthesis
of well-dispersed, electron rich, and catalytically active ultra-
small Pd NPs for FA dehydrogenation. As a result, the rate of
dehydrogenation of FA over SBA-15-Amine/Pd is much more
enhanced.
FA is known to decompose via two possible pathways: (1)
dehydrogenation (HCOOH / H2 + CO2) and dehydration
(HCOOH / H2O + CO).34 To obtain better FA dehydrogenation
systems for hydrogen production, catalysts that favor the former
and suppress the latter are unquestionably preferred. This is
Fig. 4 Recyclability results of SBA-15-Amine/Pd used in FA dehy-
drogenation: volume (mL) of gas products (H2 + CO2) versus time
(min). The inset shows a plot of initial TOFs versus catalytic recycle
runs.
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Fig. 5 TEM images of spent SBA-15-Amine/Pd catalysts after each catalytic reaction cycle.
In addition, the amount of amine in the materials aer the
6th recycle, obtained based on analysis of atomic percentage
of nitrogen on XPS (Table S2 and Fig. S7†), remains almost
unchanged.
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In conclusion, amine-functionalized SBA-15-supported
ultrasmall Pd NPs are synthesized and shown to have excellent
activity towards FA dehydrogenation at room temperature in the
´
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Acknowledgements
T.A. acknowledges the partial nancial support of the NSF
(DMR-0968937 and NanoEHS-1134289) for the work carried out
in his lab. C.W.Y. acknowledges the nancial support from the
Korea Electric Power Corporation and Korea Southern Power
Co., Ltd.
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