Investigation of Pd nanoparticles supported on zeolites for hydrogen production from formic acid dehydrogenation

Article abstract of DOI:10.1039/c4cy00667d

Catalysts based on palladium nanoparticles supported on different zeolites (BETA, ZSM-5 and Y) were prepared and their catalytic performance in formic acid dehydrogenation was studied. The effects of the zeolite structure and porous texture on the catalyt

Full text of DOI:10.1039/c4cy00667d

Catalysis
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Investigation of Pd nanoparticles supported on
zeolites for hydrogen production from formic
acid dehydrogenation
Cite this: Catal. Sci. Technol., 2015,
5, 364
a
b
a
a
M. Navlani-García, M. Martis, D. Lozano-Castelló,* D. Cazorla-Amorós,
bc
bc
K. Mori and H. Yamashita*
Received 23rd May 2014,
Accepted 20th August 2014
Catalysts based on palladium nanoparticles supported on different zeolites (BETA, ZSM-5 and Y) were
prepared and their catalytic performance in formic acid dehydrogenation was studied. The effects of the
zeolite structure and porous texture on the catalytic activity were investigated by comparing the behaviour
of these samples. The results revealed that the samples based on BETA zeolite are promising catalysts for
this application.
DOI: 10.1039/c4cy00667d
www.rsc.org/catalysis
received attention as a hydrogen source since it is nontoxic
Introduction
−
3 19
and liquid at room temperature (density, 1.22 g cm ). It is
well known that FA decomposition can take place following
two different pathways:
Nowadays, the demand for energy is getting more and more
attention, and in this sense, hydrogen is considered one of
the most promising clean energy carriers since it has several
advantages with respect to fossil fuels. The main benefits of
using H2 as an energy vector lie in the fact that it is an
inexhaustible fuel and the only product of its combustion
−1
Dehydrogenation: HCOOH ↔ H + CO , ΔG = − 48.4 kJ mol (1)
2
2
1
Dehydration: HCOOH ↔ CO + H
2
O, ΔG = − 28.5 kJ mol− (2)
1
with oxygen is water. However, one of the major drawbacks
to the use of hydrogen as fuel is its storage. For this reason, a
lot of effort is being made in order to get a suitable material
which is able to fulfill the hydrogen storage requirements.
Several kinds of materials have been considered for hydrogen
storage, either physical or chemical storage. In relation to the
physical methods, hydrogen can be stored as a compressed
As one of the main goals of hydrogen production is its use
in fuel cells, the dehydration reaction must be avoided in
order to obtain H streams free of CO (or at least with CO
2
concentration below 10 ppm) because this gas acts as a
2
0
poison for the Pt electrodes used in fuel cells, and that is
the reason why several approaches have been made to obtain
a suitable catalyst for the selective dehydrogenation of FA.
Among all the catalysts studied in this application, those
based on palladium nanoparticles seem to be very promising,
2
gas or as a cryogenic liquid, as well as by physical adsorption on
3
–5
the surface of porous solid materials, with carbon materials,
6
,7
8,9
organic polymers, zeolites, metal–organic frameworks
1
0,11
12,13
(
MOFs),
and clathrate hydrates
as the main adsor-
since a relatively high amount of H was generated and high
2
1
7,21,22
bents. Concerning the chemical storage, hydrogen storage
can take place in solid-phase (hydrides, alanates, amides and
selectivity was achieved at room temperature.
Metal
nanoparticles (NPs) are widely applied in catalysis due to
their extremely small size and high surface-to-volume ratio,
which make them very interesting candidates for use as
active phases in several catalysts. However, when this kind of
metal nanoparticles is used, some problems related to
NP aggregation may occur and this might reduce both the
1
4,15
imides, among others)
(
or liquid-phase components
1
6,17
hydrazine, alcohols and formic acid (FA), among others).
FA, which has gravimetric and volumetric hydrogen capaci-
−
1 18
ties of 4.4 wt.% and 52 g l
,
respectively, has recently
2
3
catalytic activity and the selectivity. Hence, one possible
solution for this problem could be the use of a stabilising
agent which would prevent sintering of the metal nano-
particles. The use of several stabilising agents, such as
a
Departamento de Química Inorgánica e Instituto Universitario de Materiales,
Universidad de Alicante, Ap. 99, E-03080, Alicante, Spain.
E-mail: d.lozano@ua.es; Fax: +34 96590 3454; Tel: +34 96590 3946
b
Division of Materials and Manufacturing Science, Graduate School of
24
25
26
dendrimers, tetraalkylammonium salts, or ionic liquids,
has already been reported, but polymers are one of the most
Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan.
E-mail: yamashita@mat.eng.osaka-u.ac.ps
2
7,28
c
used stabilising agents.
vinylpyrrolidone (PVP) is one of the most common combined
Among these polymers, poly-
Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University,
2
9
Katsura, Kyoto 615-8520, Japan
364 | Catal. Sci. Technol., 2015, 5, 364–371
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with different synthesis strategies, such as electrochemical
methods or reduction-by-solvent method.
changed its colour from light orange to dark brown, which
indicated that the metallic colloid had formed. Then,
3
0
31
40
Several kinds of materials have been used as catalyst
supports of metallic nanoparticles in FA decomposition
heating was maintained for 2 h, after which the bath was
removed and the colloidal suspension was cooled down to
room temperature.
2
1
32
33
22
(i.e. macroreticular resin, graphene oxide, amines, MOFs
and so on). Although zeolites constitute one of the most
important groups of heterogeneous catalysts supports with
Once the colloids were prepared, the palladium nano-
particles were purified and redispersed in methanol. The
purification was carried out by pouring an aliquot of nano-
particle colloid into a glass bottle, adding an excess of
acetone and shaking the solution. As we have previously
reported, the treatment with acetone caused the extraction
of the protecting polymer to the acetone phase and floccula-
tion of the palladium nanoparticles. Then, the supernatant
organic phase was removed and the purified nanoparticles
were redispersed by stirring in a known amount of methanol,
so that the final suspension concentration was perfectly known.
3
4
large-scale applications, to our knowledge, their use in this
application has not been reported yet. These materials are
crystalline aluminosilicates having unique properties, such as
thermal stability, relatively high surface area, intrinsic acidity,
defined microporosity and ion exchange capacity, which
make them suitable materials in different applications.
There are many different framework structures. The
supports studied in this work are BETA zeolite (Framework
Type *BEA with 3-dimensional channels and a pore system
consisting of 12-membered rings with two opening channel
diameters of 0.56 × 0.56 and 0.77 × 0.66 nm), ZSM-5 zeolite
3
9
Catalysts support preparation
(
Framework Type MFI with 3-dimensional channels and a
Four catalyst supports consisting of the acid zeolites BETA,
ZSM-5 and Y were studied. The acid zeolites BETA and ZSM-5
pore system consisting of 10-membered rings with opening
channel diameters of 0.51 × 0.55 and 0.53 × 0.56 nm) and Y
zeolite (Framework Type FAU with 3-dimensional channels
and a pore system consisting of 12-membered rings with an
opening channel diameter of 0.74 × 0.74 nm).
them, it is well known that BETA zeolite is a promising
(
H-BETA and H-ZSM-5) were obtained directly by conversion
of commercial zeolites (NH -BETA and NH -ZSM-5 with Si/Al =
2.5 and 15.0, respectively, supplied by Zeolyst International)
4
4
1
3
5,36
Among
by heat treatment in air at 450 °C for 6 h, with a heating rate
−1
of 5 °C min . On the other hand, the acid counterpart of the
3
7,38
heterogeneous catalyst for several liquid-phase reactions.
Y zeolite was obtained by ion exchange of the commercial
sodium form (Na-Y, with Si/Al = 2.5, supplied by Sigma
Aldrich). For this purpose, the sodium zeolite was ion
Considering all the abovementioned, the goal of the present
work is to prepare catalysts based on PVP-stabilised palladium
nanoparticles supported on different zeolites (BETA, ZSM-5
and Y) and to study their performance in the formic acid
dehydrogenation reaction at low temperature.
exchanged with 1 M NH
.25 g/100 ml and maintaining stirring in a thermostatic bath
at 60 rpm at 30 °C for 24 h. Afterwards, the NH -Y zeolite was
heat treated in air at 550 °C for 4 h with a heating rate of
4 3
NO , using a zeolite/solution ratio of
0
4
−
1
Experimental
Synthesis and purification of nanoparticles
1 °C min in order to obtain the final H-Y support. After ion
exchange, the sample was filtered, washed with distilled
water and dried at 110 °C.
Palladium nanoparticles were synthesised in an inert argon
atmosphere by following the reduction by solvent method as
reported by our research group elsewhere.
Catalysts preparation
3
1,39
The palladium
The catalysts studied in this work were prepared by impreg-
nation of the zeolitic supports with the palladium nano-
precursor was Pd(ac) (98%, Sigma-Aldrich) and the stabilising
2
agent used was polyvinylpyrrolidone (PVP, 40 K, Sigma-
Aldrich). The metal nanoparticles were synthesised by the
following experimental procedure.
For solution 1, 1.114 g (10 mmol) of PVP (stabilising
agent) were added to 120 ml of ethylene glycol in a three-
necked round-bottomed flask and the solution was stirred at
31
particle colloid following a previously reported methodology.
The impregnation was carried out by mixing a known amount
of each support with an adequate amount of the purified
nanoparticle colloid suspension in order to obtain a final
metallic loading of 1 wt.%. The mixtures were vigorously
stirred for 48 h using a magnetic stirrer to guarantee similar
metal loading and distribution in all the catalysts and then
the samples were heat treated at 60 °C to remove the solvent
8
0 °C for 2 h using a magnetic stirrer and an oil bath.
For solution 2, 0.2245 g (1 mmol) of Pd(ac) were dissolved
2
in 50 ml of dioxane by stirring for 2 h at room temperature
using a magnetic stirrer. The colour of this solution was light
orange.
After the solution preparation, solution 1 was cooled down
to 0 °C in an ice bath and 3 ml of 1 M NaOH solution was
added under stirring in order to adjust the pH of the
resulting mixture. Then, solution 2 was poured into solution
(
methanol). Finally, the catalysts were washed several times
with a cold mixture of H O/EtOH (50 : 50, v/v) and dried at
0 °C for 12 h. The resulting catalysts were denoted as
2
6
Pd/H-BETA, Pd/H-ZSM-5 and Pd/H-Y.
Catalysts characterisation
1
under vigorous stirring in order to ensure homogenisation
The textural characterisation of the zeolitic supports was
and the final mixture was heated up to 100 °C. The solution
carried out by means of adsorption of N at −196 °C and CO
2
2
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at 0 °C (Autosorb 6, Quantachrome). Prior to the adsorption
measurements, the supports were outgassed in situ under
vacuum at 250 °C for 4 h in order to remove any adsorbed
impurities. Apparent surface area values were calculated from
nitrogen adsorption isotherms using the BET equation (SBET).
acid for 72 h. The samples were weighted before and after
the formic acid adsorption, thus the increase in weight indi-
cated that in all cases the formic acid adsorption had taken
place due to its high vapor pressure. Afterwards, samples
were placed in the spectrometer chamber in the presence of
Total micropore volume (VDR(N
2
)) and narrow micropore
2 2
a dry air flow (CO and H O free) and the spectra were recorded
volume (VDR(CO )) were calculated by applying the Dubinin–
at room temperature.
2
Radushkevich (DR) equation to the N2 adsorption data at
−
196 °C and the CO
2
adsorption data at 0 °C, respectively.
(N )) was also calculated from the N
2 2
Catalytic tests of formic acid dehydrogenation
Mesopore volume (V
meso
In order to analyse the catalytic performance of the different
samples, 50 mg of each powder sample and 9.6 ml of water
were mixed in a Pyrex reaction vessel (30 ml) which was
sealed with a rubber septum. Afterward, samples were treated
in an ultrasound bath for 15 min for complete dispersion of
the zeolitic samples. Then, the mixture was bubbled with
argon gas for 30 min to ensure an inert atmosphere. Once
this mixture was prepared, 0.39 ml of FA was added into the
vessel and it was placed in an oil bath at 50 °C with magnetic
stirring for 3 h. The subsequent H and CO generation was
isotherm taking into account the volume adsorbed between
the partial pressures 0.9 and 0.2 in the desorption branch.
External zeolite surface area was calculated by applying the
t-plot method.
Transmission electron microscopy (TEM) images were
obtained by using a JEOL (JEM-2010) equipped with an EDS
analyzer (Oxford, model INCA Energy TEM 100) operating at
2
00 kV with a space resolution of 0.24 nm. In order to prepare
the samples for the TEM analysis, a small amount of the cata-
lyst was suspended in methanol and sonicated in an ultrasonic
bath for a few minutes. After that, a drop of this suspension
was deposited onto a 300-mesh Lacey copper grid and dried at
room temperature. TEM images were also captured for the
palladium colloid in order to compare the NP diameter and
particle size distribution before and after the impregnation step.
The dispersion of the palladium nanoparticles on the
catalyst (D), which is an estimation of the amount of metal
exposed in the nanoparticles, was calculated from the following
equation, assuming a spherical shape of the particles (d, particle
2
2
monitored by using a Shimadzu GC14B equipped with an
MS5 A column (H ) and a Shimadzu GC (CO ). For this
2
2
purpose, injections in both GC were made and the resulting
peak areas were converted into H and CO concentrations by
2
2
using a calibration with the corresponding standard gases.
Results and discussion
Characterisation
4
1
Fig. 1 shows the N2 adsorption–desorption isotherms at
diameter):
−
196 °C for all the zeolitic supports. ZSM-5 and Y zeolites
present type I isotherms according to the IUPAC classifica-
D (%) ≈ (0.9)/d (nm)
4
2
tion, showing a high uptake of nitrogen at very low relative
pressures, which is characteristic of microporous materials.
BETA zeolite has a type I+IV isotherm, with a high uptake
of nitrogen at very low relative pressures and a hysteresis
loop because of the presence of micro- and mesoporosity.
Table 1 summarizes the BET surface area (SBET), the total
Inductively coupled plasma-optical emission spectroscopy
ICP-OES) was the selected technique to determine the palla-
4
2
(
dium loading. For this purpose, a Perkin Elmer Optima 4300
system was used. The palladium extraction was carried out
by oxidative treatment with aqua regia for 48 h. Then, the
samples were filtered and diluted.
micropore volume (VDR(N )), the narrow micropore volume
2
2 2
(VDR(CO )), the mesopore volume (Vmeso(N )) and the external
Pd K-edge XAFS spectra were recorded using
a
fluorescence-yield collection technique at the beam line 01B1
station with an attached Si (311) monochromator at SPring-8,
JASRI, Harima, Japan (prop. no. 2013B1041). The EXAFS data
were normalized by fitting the background absorption coeffi-
cient around the energy region higher than the edge of about
3
5–50 eV, with the smoothed absorption of an isolated atom.
The EXAFS data were examined using the Rigaku EXAFS anal-
ysis program.
Moreover, in order to analyse the interaction between the
formic acid and the different zeolites, DRIFT spectra of
zeolitic supports and catalysts before and after formic acid
adsorption were recorded by using a FTIR spectrometer
(
Jasco, model FT/IR-4100). Prior to the DRIFT measurement,
the adsorption of formic acid was carried out at room tem-
perature. For this purpose, the samples were placed in a dry
chamber (free of humidity) with a vessel containing formic
Fig. 1
N
2
adsorption–desorption isotherms at −196 °C corresponding
to H-BETA, H-ZSM-5 and H-Y zeolites.
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Table 1 Porous texture characterisation results
2
−1
3
−1
3
−1
3
−1
2
−1
Sample
S
BET (m g
)
VDR(N
2
) (cm g
)
V
DR(CO
2
) (cm g
)
V
meso (cm g
)
External surface area (m g
)
H-BETA
H-ZSM-5
H-Y
590
370
725
0.27
0.16
0.35
0.25
0.15
0.41
0.21
0.03
0.01
122
42
52
zeolite surface area of the catalyst support. As can be
observed from Table 1, the zeolite specific surface area (SBET
)
follows the order Y > BETA > ZSM-5. The total micropore
volume (VDR(N )) calculated from the nitrogen isotherm and
2
2
the narrow micropore volume (VDR(CO )) calculated from the
CO2 isotherm follow the same tendency. The mesopore
volume (Vmeso) is much higher for the BETA zeolite than for
the other supports, which is in good agreement with the
shape of the nitrogen isotherms, where a more important
hysteresis loop was observed for that zeolite. Moreover, a very
important difference was observed for the external surface
area determined for the catalyst support since the H-BETA
zeolite showed a much higher external surface than those
observed for H-ZSM-5 and H-Y as a consequence of its char-
4
3,44
acteristic surface roughness.
Fig. 2 shows a TEM micrograph of the palladium nano-
particle colloid and the histogram with the particle size dis-
tribution for this colloid. The analysis of the TEM images for
the colloid indicated that the Pd NPs show a small average
particle size (d = 2.1 ± 0.3 nm) and a very narrow size distri-
3
1,45
bution, as was previously reported by our research group.
Moreover, as can be observed, palladium nanoparticles pres-
ent a spherical shape in the as-prepared colloid.
Fig. 3 includes the TEM images of the as-prepared cata-
lysts and the pertinent histograms with the particle size
distributions. From the TEM images observation, it can be
concluded that the NPs supported on the different zeolites
show a very narrow size distribution and aggregates of palla-
dium nanoparticles are not formed as a consequence of the
impregnation or drying steps. However, a slight loss of their
spherical shape can be appreciated, which might be favoured
by the interaction between the metal and the zeolite surface.
Table 2 summarizes the nanoparticle sizes for all the studied
samples, the dispersion calculated from the TEM images
Fig. 3 TEM images and the corresponding histograms of samples (A)
Pd/H-BETA, (B) Pd/H-ZSM-5 and (C) Pd/H-Y.
and the metal loading determined by means of ICP. Metal
loading, determined by means of ICP-OES (see Table 2),
achieved after 48 h of impregnation is very close to 1 wt.%
for all the samples.
Table 2 shows that an important particle size increase is
produced when the palladium nanoparticles are deposited
onto the different zeolites, especially in the case of the sample
Pd/H-Y, where the particle size almost doubles the size mea-
sured in the metal colloid (d = 2.1 ± 0.3 nm). This size
increase could be a consequence of the impregnation
Fig.
2 TEM image and histogram with the nanoparticle size
distribution of the palladium colloid.
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Table 2 Particle size (dTEM), dispersion (DTEM) and metal loading of the
catalysts
Sample
dTEM (nm)
DTEM (%)
Pd loading ICP (wt.%)
Pd/H-BETA
Pd/H-ZSM-5
Pd/H-Y
3.4 ± 0.5
3.0 ± 0.6
4.1 ± 0.9
26.5
30.0
22.0
1.0
0.9
0.9
process, where some nanoparticle deformation can take place
leading to larger metal nanoparticles. Considering the nano-
particle size and the zeolite pore openings, it could be said
that in all the samples the palladium nanoparticles are
located on the external zeolite surfaces.
3
Fig. 4 shows the Fourier transform (FT) k -weighted Pd
K-edge extended X-ray absorption fine structure (EXAFS) of
these samples. All samples display a main peak centered at
2
.65 Å, which is assigned to a single Pd–Pd bond, suggesting
that Pd nanoparticles exist in a metallic form.
For a better understanding of the catalytic performance in
the formic acid decomposition reaction, the interaction
between formic acid and the different samples has been
studied by DRIFT experiments. DRIFT spectra were recorded
before and after formic acid adsorption, and Fig. 5 contains
the DRIFT spectra for the catalysts Pd/H-BETA, Pd/H-ZSM-5
and Pd/H-Y after subtraction of the spectrum before formic
acid adsorption. Therefore, the appearance of a positive peak
means that this peak appears as a consequence of the inter-
action between the sample and the formic acid, while the
negative peaks indicate that these peaks have been removed
from the original sample spectrum after this adsorption.
Moreover, it should be highlighted that only the spectral
−
1
region from 1400 to 4000 cm is shown, since, although Pd
−
1
surfaces give rise to an absorption band at ~1350 cm
,
in our case the peaks obtained in this region led to a very
complicated spectrum in which the contribution of formic
acid was not clearly elucidated. This was due to the fact that
the studied palladium nanoparticles are not only consti-
tuted by Pd but also by a polymeric capping agent, poly-
vinylpyrrolidone (PVP), which shows typical vibrational
Fig. 5 DRIFT spectra (after subtraction of the spectrum before formic
acid adsorption) for the catalysts (A) Pd/H-BETA, (B) Pd/H-ZSM-5 and
−
1 45
bands between 1300 and 1660 cm
,
and these bands
(C) Pd/H-Y.
overlap with those caused by the formic acid adsorption on
the palladium surface.
It can be observed from Fig. 5 that for all the samples,
after formic acid adsorption, two new peaks at about
−
1
−1
~
1700 cm and ~2940 cm appear, which correspond to the
4
6
ν(CO) and ν(CH) of the formic acid, respectively. Com-
paring the relative intensities of these peaks, it can be seen
that the lowest intensity peaks correspond to the catalyst
based on ZSM-5 zeolite, which indicates that this material
has the lowest formic acid adsorption capacity in concor-
dance with its lower specific surface area (see Table 1). Other
relevant spectroscopic features observed after the formic acid
adsorption are in the OH group region. In this sense, the
exposure of the surface of the samples based on BETA and
Fig. 4 Pd K-edge FT-EXAFS spectra for (a) Pd/H-BETA, (b) Pd/H-Y and
c) Pd/H-ZSM-5.
(
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ZSM-5 zeolites results in the adsorption of the formic acid
molecules on the zeolite surface with a loss of surface –OH
groups, which is confirmed by the negative intensities of
the peak attributed to the O–H stretching of free silanols
mainly located on the external surface, which is centered
Catalytic tests of formic acid dehydrogenation
Dehydrogenation of formic acid was performed at 50 °C for
all the as-prepared catalysts and the progress of the reaction
was monitored by quantifying the hydrogen output after fixed
times.
Fig. 7 shows the amount of hydrogen produced from the
formic acid decomposition at 50 °C by using the acid cata-
lysts Pd/H-BETA, Pd/H-ZSM-5 and Pd/H-Y.
Fig. 7 shows that the catalytic activity is different for all
the samples. In this sense, the sample Pd/H-BETA shows the
best catalytic performance, generating 135.6 μmol of H
−
1
47,48
at 3750 and 3700 cm , respectively.
Several studies
analysing the adsorption of formic acid on different materials
4
6,49
have already been reported in the literature.
Regarding
the adsorption on OH groups, in a theoretical study carried
5
0
out by Iuga et al. where the adsorption of formic acid
molecules on different silicate surface models were included, it
was reported that the most stable configuration is the one in
which the carbonyl O atom interacts with a silanol group, while
the hydroxyl H atom of HCOOH interacts with the O atom of
the siloxane Si–O–Al bridge by forming a hydrogen bond in
both cases. Thus, if this adsorption geometry is extrapolated to
the formic acid adsorption carried out in the present work, the
zeolite surface –OH groups would be hydrogen-bonded to
the formic acid and that could explain the broad band centered
2
after
3
h of reaction at 50 °C. Samples Pd/H-ZSM-5 and Pd/H-Y
2
displayed similar H production profiles, although the final
H generated after 3 h is slightly higher in the case of sample
2
Pd/H-ZSM-5 (72.4 and 64.2 μmol of H , respectively). In order
2
to confirm that the dehydrogenation of formic acid does not
proceed on bare zeolites, control experiments with the bare
zeolitic supports (H-BETA, H-ZSM-5 and H-Y) were carried
out, and as expected, no catalytic activity was observed for
any of them.
−1
at 3269 and 3370 cm for samples based on BETA and Y zeolites,
4
6
respectively. Fig. 6 shows a schematic representation of the
Taking into account that all the catalysts have similar
metal content and similar nanoparticle size, it seems that the
zeolite structure and porous texture play important roles in
the catalytic performance. In this sense, since the palladium
nanoparticles are located on the external zeolite surface, the
distribution of the nanoparticles and the stabilising effect of
the zeolite are determined by their external zeolite surface.
Thus, the higher external surface of the BETA zeolite (see
Table 1), which is a consequence of its characteristic surface
roughness, leads to a better palladium nanoparticle distribution
in comparison with those obtained for ZSM-5 and Y zeolites,
which reflects in the paramount catalytic performance displayed
by sample Pd/H-BETA.
4
6
interaction between formic acid and zeolites.
In summary, the changes observed in the OH region are
a consequence of two different contributions. On the one
hand, the adsorbed formic acid molecules on the zeolite
surface produce the break of some –OH bonds of the free
silanols (see contribution (a) in Fig. 6), which reflects in the
−
1
negative bands at 3750 and 3700 cm for Pd/H-BETA and
Pd/H-ZSM-5, respectively. On the other hand, the formic acid
molecules can interact with the zeolite through the hydroxyl
H atom of the HCOOH molecule, which interacts with the
O atom of the Si–O–Al bridge (see contribution (b) in Fig. 6),
which reflect in the positive broad band centered at 3269 and
−
1
3
370 cm for samples Pd/H-BETA and Pd/H-Y zeolites,
Moreover, the DRIFT spectra analysis suggests that there
is an interaction between the acid sites of the zeolites and
the formic acid molecules, which seems to be favoured in the
case of sample Pd/H-BETA in comparison with Pd/H-ZSM-5
and Pd/Y samples. This interaction could lead to an
respectively.
Bearing all these considerations in mind and considering
the relative intensities of the peaks and bands of the differ-
ent spectra, it could be elucidated that the strength of the
interaction between the formic acid molecules and the zeolite
surface is different for the studied samples. It seems that this
interaction is favoured in the case of the sample based on
BETA and Y zeolites and is less important for the ZSM-5
zeolite-based sample.
Fig. 7 Evolution of the hydrogen production from formic acid
dehydrogenation at 50 °C using Pd/H-BETA, Pd/H-ZSM-5 and Pd/H-Y
catalysts.
Fig. 6 Schematic representation of the interaction between formic
acid and zeolites.
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Table 3 Palladium loading, temperature used in the formic acid dehydrogenation reaction and hydrogen produced in that reaction after 3 h for the dif-
ferent catalysts studied
Sample
Pd loading ICP (wt.%)
Temperature (°C)
2
H produced (μmol, t = 3 h)
Pd/H-BETA
Pd/H-BETA
Pd/H-BETA(0.5)
1.0
1.0
0.5
50
30
50
135.6
99.1
111.1
activation of the formic acid molecules and the subsequent
catalytic activity improvement. The interaction between acid
sites and formic acid molecules could also increase the
surface concentration of formic acid, which would be more
important in the case of the Pd/H-BETA sample.
To check the effect of temperature on the catalytic perfor-
mance of the Pd/H-BETA, the same reaction was carried out
at 30 °C. As expected, the hydrogen production achieved after
for hydrogen production from formic acid dehydrogenation.
The results suggested that the porous structure of the zeolite
plays an import role in the catalytic performance, being
better in the case of samples based on BETA zeolite. The
presence of higher external surface in BETA compared to
those values obtained for ZSM-5 and Y zeolites helped to
obtain well-distributed Pd nanoparticles. This fact, together
with the high interaction between the acid sites of BETA
zeolite and the formic acid molecules, as deduced from
DRIFT spectra analysis, allowed us to obtain a very promis-
ing catalyst for hydrogen production from formic acid
decomposition.
3
h is slightly lower compared with the one obtained when
the reaction was followed at 50 °C (see Table 3) but still quite
high.
For comparison purposes with previous results obtained
by our group, a catalyst based on Pd/H-BETA with a lower
palladium loading (0.5 wt.%; sample denoted as Pd/H-BETA(0.5))
was prepared and tested in the formic acid decomposition
reaction (see results in Table 3). Comparing these results
with those previously reported by our research group, the out-
standing catalytic performance of the sample based on the
BETA zeolite is demonstrated. In this sense, when the catalyst
Acknowledgements
The authors would like to acknowledge the Ministerio de
Economía y Competitividad, GV and FEDER (CTQ2012/31762
and PROMETEOII/2014/010) for the financial support. Miriam
Navlani-García thanks the University of Alicante for the PhD
studentship. Martin Martis thanks the Japan Society for the
Promotion of Science (JSPS) for the fellowship.
2
2
was based on Pd nanoparticles supported on MOF, the
amount of hydrogen produced after 3 h of reaction (under
almost the same experimental conditions) was much lower
than the amount of hydrogen generated by using sample
Pd/H-BETA(0.5) (48.1 and 99.1 μmol, respectively). Moreover,
the hydrogen produced by using the sample Pd/H-BETA(0.5)
is slightly higher than that produced by using a more complex
material based on Pd nanoparticles within a macroreticular
basic resin with the same metal loading (111.1 and 96 μmol,
respectively) and very similar to those achieved when the cat-
alytic activity of this macroreticular basic resin is enhanced
by adding Ag in Pd/Ag molar ratios of 1 : 1 (Pd 0.5 wt.% and
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