Hydrolytic dehydrogenation of ammonia borane catalyzed by reduced graphene oxide supported monodisperse palladium nanoparticles: High activity and detailed reaction kinetics

Article abstract of DOI:10.1016/j.molcata.2012.05.008

A highly active and stable catalyst for the hydrolytic dehydrogenation of ammonia borane (AB) was prepared by supporting monodisperse palladium nanoparticles (Pd NPs) on reduced graphene oxide (RGO) via a facile method. RGO was prepared via modified chemical route and used as support matrices for monodisperse Pd NPs that were formed by the reduction of palladium(II) acetylacetonate by borane tert-butylamine complex in the presence of oleylamine. RGO supported Pd NPs (RGO@Pd) show high activity and stability in the hydrolytic dehydrogenation of AB. The RGO@Pd catalysts provide the turnover frequency of 26.3 min^-1 - the best among the all Pd-based catalysts and even comparable to Pt-based catalysts tested in the hydrolysis of AB. They are also very stable catalysts providing 11,600 total turnovers in 46 h. The detailed reaction kinetics of catalytic hydrogen generation from the hydrolysis of AB revealed that the reaction proceeds first order with respect to the Pd concentration and zeroth order with respect to the AB concentration. The apparent activation parameters of the catalytic hydrolysis reaction were also calculated; apparent activation energy (Eaapp)=402kJmol^-1, activation enthalpy (ΔH^#,app) = 38 ± 1 kJ mol^-1 and activation entropy (ΔS^#,app) = -134 ± 1 J K^-1 mol^-1.

Full text of DOI:10.1016/j.molcata.2012.05.008

Journal of Molecular Catalysis A: Chemical 361–362 (2012) 104–110
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Hydrolytic dehydrogenation of ammonia borane catalyzed by reduced graphene
oxide supported monodisperse palladium nanoparticles: High activity and
detailed reaction kinetics
Buket Kılıc¸ a, Selin S¸ encanlıb, Önder Metina,∗
a Department of Chemistry, Faculty of Science, Atatürk University, 25240 Erzurum, Turkey
b Department of Chemistry, Middle East Technical University, 06800 Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
A highly active and stable catalyst for the hydrolytic dehydrogenation of ammonia borane (AB) was
prepared by supporting monodisperse palladium nanoparticles (Pd NPs) on reduced graphene oxide
(RGO) via a facile method. RGO was prepared via modified chemical route and used as support matri-
ces for monodisperse Pd NPs that were formed by the reduction of palladium(II) acetylacetonate by
borane tert-butylamine complex in the presence of oleylamine. RGO supported Pd NPs (RGO@Pd) show
high activity and stability in the hydrolytic dehydrogenation of AB. The RGO@Pd catalysts provide the
turnover frequency of 26.3 min−1—the best among the all Pd-based catalysts and even comparable to
Pt-based catalysts tested in the hydrolysis of AB. They are also very stable catalysts providing 11,600
total turnovers in 46 h. The detailed reaction kinetics of catalytic hydrogen generation from the hydroly-
sis of AB revealed that the reaction proceeds first order with respect to the Pd concentration and zeroth
order with respect to the AB concentration. The apparent activation parameters of the catalytic hydrolysis
reaction were also calculated; apparent activation energy (Eaapp) = 40 2 kJ mol−1, activation enthalpy
Received 9 February 2012
Received in revised form 1 May 2012
Accepted 5 May 2012
Available online 14 May 2012
Keywords:
Reduced graphene oxide
Palladium nanoparticles
Supported catalysts
Ammonia borane
Dehydrogenation
(ꢀH#,app) = 38 1 kJ mol−1 and activation entropy (ꢀS#,app) = −134 1 J K−1 mol−1
.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
thermolysis and slow reaction kinetics of other solvolysis routes
[17,18].
The storage of hydrogen is still a major hurdle in front of the
hydrogen economy although the use of hydrogen as an energy
carrier in various daily-life applications has been already initiated
[1,2]. Many methods have been proposed for the storage of hydro-
gen but none of them is a solution by itself [3]. One conceptual
solution is the generation of hydrogen from chemical compounds
having high gravimetric and volumetric hydrogen storage capac-
ity such as boron–nitrogen compounds [4–6]. Recently, ammonia
borane (AB, H3N·BH3) is considered to be the most promising can-
didate for on-board hydrogen storage applications among all other
chemical hydrogen storage materials owing to its remarkable gravi-
metric hydrogen content of 19.6 wt%, stability in the solid state
and solution under ambient conditions [7–9]. The hydrogen stored
in AB can be released through mainly two ways; thermolysis and
solvolysis [10–12]. However, the dehydrogenation of AB in water
(hydrolysis, Eq. (1)) [13–16] seems to be the most promising route
for on-board applications due to high temperature requirement of
H3NBH3(aq) + 2H2O (l) → (NH4)BO2(aq) + 3H2(g)
(1)
However, hydrolysis of AB is a harsh reaction because of the
strong B N bond of AB complex and therefore a catalyst facilitating
the B N bond cleavage is required to generate hydrogen at desired
rate [19,20]. Although many transition metal catalysts or their
alloys have been studied to generate hydrogen from the hydrol-
ysis of AB [21–25], a prosperous catalyst that meets performance,
cost and safe requirements as well as the enhanced kinetic proper-
ties under moderate conditions still needs to be developed for the
practical application of this system. Actually, the development of
an efficient heterogeneous catalyst is aspired for the hydrolysis of
AB considering the advantageous of heterogeneous catalysis over
the homogeneous one such as reusability and bottleability of the
catalyst [26,27]. An efficient way of increasing catalytic activity of
a heterogeneous catalyst is the use of transition metal nanopar-
ticles (NPs) which are more active catalysts than the respective
bulk metal owing to their high surface-to-volume ratio resulting in
larger fraction of catalytically active atoms on their surface [28–30].
However, one of the most important problems in their catalytic
applications is the aggregation of nanoparticles into clumps and
ultimately to the bulk metal, which leads to a vital decrease in their
∗
Corresponding author. Tel.: +90 442 2314410, fax: +90 442 2360948.
E-mail address: ondermetin1981@hotmail.com (Ö. Metin).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.05.008

B. Kılıc¸ et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 104–110
105
NMR). BF3.(C2H5)2O was used as the external reference for 11B NMR
chemical shifts.
catalytic activity and lifetime [31,32]. One of the most promising
ways to enhance the stability and reusability of transition metal NPs
is supporting them on the suitable inorganic solid matrices [33,34].
Recently, reduced graphene oxide (RGO) has emerged as a practical
support material for various metal NPs [35–38] owing to its huge
surface area and excellent chemical stability [39]. Among them, Pd
NPs is especially important due to their extensive catalytic use in
various industrial reactions including hydrolysis of AB [40–42]. In
this regard, the preparation of Pd NPs via controlled way and sup-
porting them on RGO would be advantageous for the preparation
of the desired heterogeneous catalyst for the hydrolysis of AB.
Herein we report a facile method for the preparation of RGO
supported Pd NPs (RGO@Pd) and their excellent catalysis in the
hydrolytic dehydrogenation of AB. To the best of our knowledge,
this is the first example of the RGO@Pd catalysts which are gen-
erated by supporting ex situ prepared Pd nanoparticles via simple
liquid phase impregnation on RGO and their use as heterogeneous
catalyst in the hydrolytic dehydrogenation of AB. RGO@Pd catalyst
provided the highest activity in terms of initial turnover frequency
(TOF) of 26.3 min−1 among the all Pd-based catalysts as well as its
stability (11,600 total turnovers in 46 h) in the hydrolytic dehydro-
genation of AB. They are also durable catalysts in the hydrolytic
dehydrogenation of AB preserving their 95% initial activity after
10th run. Additionally, the present study includes a detailed kinetic
study on catalytic hydrogen generation from the hydrolysis of AB
that is studied depending on the catalyst concentration, substrate
concentration and temperature, which will give a physical insight
to readers.
2.3. Synthesis of graphite oxide, reduced graphene oxide and
monodisperse palladium nanoparticles
The preparation of graphite oxide and reduced graphene oxide
was reported elsewhere [43]. Pd NPs were prepared using a modi-
fied version of the oleylamine mediated synthesis [44]. In a typical
synthesis, 0.25 mmol palladium(II) acetylacetonate and 12.0 mL
of OAm were mixed and stirred at 1000 rpm under continuous
nitrogen flow in a special four-necked reactor. Owing to jacketed
heater and connected thermocouple placed into the reactor, the
mixture was heated up to 70 ◦C slowly. Next, 350 mg of borane tert-
butylamine complex dissolved in 32.0 mL of OAm was injected into
the system when the temperature settled down at 70 ◦C. Next, the
resulted mixture was heated up to 95 ◦C expeditiously and the sys-
tem was led to stay at this temperature for 1 h. Then, the resulted
solution was cooled down to 40 ◦C and centrifuged at 9000 rpm
for 12 min after the addition of ethanol into the each nanoparticle
solution separated into four centrifuge tubes.
2.4. Method for supporting palladium nanoparticles on reduced
graphene oxide
Accurate weight of Pd NPs dispersed in hexane was calculated
via gravimetric method before their impregnation on RGO and
desired amount of them was mixed with dried RGO to ensure the
ratio of 3% Pd (w/w). Then, the mixture was diluted with ∼5–10 mL
of hexane and stirred overnight. After evaporating hexane in rotary
evaporator, the dry solid was transferred into a schlenk tube. The
resulted powder of RGO@Pd was stored for using as catalyst in the
dehydrogenation and hydrolysis of AB. The palladium contents of
RGO@Pd samples were determined as 2.1 wt% by ICP-OES and used
for the calculation of catalyst concentration in all kinetic studies.
2. Experimental
2.1. Materials
Oleylamine (OAm, >70%), palladium(II) acetylacetonate
(Pd(acac)2, 99%), borane ammonia complex (AB, 97%), hexane
(99%), borane tert-butylamine complex, potassium perman-
ganate (KMnO4), hydrogen peroxide (H2O2, 30%), sodium nitrate
(NaNO3), sulfuric acid (H2SO4, 98%), hydrazine hydrate were
purchased from Sigma–Aldrich® and used as received. Natural
graphite flakes (average particle size 325 mesh) were purchased
from Alfa Aesar and used without any further treatment. Deionized
water was distilled by water purification system (Milli-Q System).
All glassware and Teflon-coated magnetic stir bars were cleaned
with acetone, followed by copious rinsing with distilled water
before drying at 150 ◦C in oven for overnight.
2.5. Hydrolysis of ammonia borane catalyzed by reduced
graphene oxide supported palladium nanoparticles
Desired amount of RGO@Pd catalyst (2.1 wt% Pd) was dis-
persed in 7.0 mL of water in the jacketed reactor thermostated at
25.0 0.5 ◦C. Next, 2.0 mmol AB dissolved in 3.0 mL of water was
injected into the catalyst solution via gastight syringe under vig-
orous stirring and hydrogen gas evolution started immediately.
The catalytic hydrolysis reaction was followed by measuring the
hydrogen generation with time. Hydrogen gas generation from the
catalytic reaction solution was followed by using a typical water-
filled gas burette system and recording the displacement of water
level in the gas burette every minute until no more hydrogen evolu-
tion observed. Next, an approximately 0.1 mL aliquot of the reaction
solution in the reactor was withdrawn with a glass Pasteur pipette
and added to 0.5 mL of D2O in a quartz NMR sample tube (Norell S-
500-QTZ), which was subsequently sealed. The 11B NMR spectrum
of this solution showed the complete conversion of H3NBH3 (quar-
tet at −23 ppm) to ammonium metaborate giving a singlet peak at
9.0 ppm.
2.2. Instrumentation
Samples for transmission electron microscope (TEM) and high
resolution TEM (HRTEM) analyses were prepared by depositing a
single drop of sonificated Pd NPs or RGO@Pd catalyst dispersed in
hexane on amorphous carbon coated copper grids. Images were
obtained by a JEOL 2100 TEM (200 kV). SEM images were acquired
using a Zeiss EVO40 environmental SEM that is equipped with a
LaB6 electron gun, a vacuum SE detector, an elevated pressure SE
detector, a backscattering electron detector (BSD), and a Bruker
AXS XFlash 4010 detector. Palladium content of the RGO@Pd sam-
ples were determined by using Leamen series inductively coupled
plasma-optical emission spectroscopy (ICP-OES) instrument after
each catalyst sample was completely dissolved in the mixture of
HNO3/HCl (1/3 ratio). Brunauer–Emmer–Teller (BET) surface area
analysis is performed using a NovaWin2 system (Quantachrome)
after degassing all of the samples for 24 h at 80 ◦C. 11B NMR spec-
trum was measured on a Bruker Avance DPX 400 MHz spectrometer
2.6. Kinetics of hydrolysis of ammonia borane catalyzed by
reduced graphene oxide supported monodisperse palladium
nanoparticles
In order to establish the rate law for the hydrolysis of AB
catalyzed by RGO@Pd, three different sets of experiments were per-
formed for each of these catalysts in the same way as described in
Section 2.5. In the first set of experiments, the concentration of
AB was kept constant (200 mM) and the amount of RGO@Pd was
(400.1 MHz for 1H NMR; 100.6 MHz for 13C NMR; 128.2 MHz for 11
B

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B. Kılıc¸ et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 104–110
3. Results and discussion
3.1. Preparation of monodisperse palladium nanoparticles and
support them on reduced graphene oxide
Monodisperse palladium nanoparticles were synthesized from
the reduction of palladium(II) acetylacetonate by borane tert-
butylamine complex in the presence of oleylamine (OAm) using a
modified oleylamine mediated synthesis [44]. The detailed charac-
terization of monodisperse Pd NPs can be found in one of our recent
report [43]. Next, the support material RGO was prepared by a two-
step procedure including the combination of a modified Hummer’s
method for the preparation of graphite oxide [45] and concomitant
hydrazine hydrate reduction of graphite oxide in aqueous solution
[39]. Fig. 1 shows a representative SEM image of RGO in which the
typical thin layered and transparent structure of RGO can nicely be
seen. In addition to the microscopic examination of RGO, its specific
surface area was found to be 500 m2 g−1 by using the BET nitrogen
adsorption analyzer.
Fig. 1. A representative SEM image of RGO.
As the next, Pd NPs were supported on RGO via simple liquid
impregnation method. Fig. 2 shows typical TEM images of RGO@Pd
catalyst at different magnifications taken from the hexane disper-
sion. As clearly seen from Fig. 2, Pd NPs are well dispersed on RGO
and preserved their particle size distribution without any formation
of clumps.
The ICP-OES analyses performed on different RGO@Pd catalyst
samples revealed that they consist of 2.1 wt% Pd and this value was
used for the calculation of all catalyst concentrations in the kinetic
study.
Powder-XRD and HR-TEM analyses were used to investigate the
crystallinity of the Pd NPs supported on RGO. Fig. 3a shows the XRD
pattern of the as-prepared RGO@Pd catalyst. Three reflections at
2ꢁ = 39◦, 42◦ and 72◦ attributable to (1 1 1), (2 0 0) and (3 1 1) planes
of a face centered cubic (fcc) crystal structure of palladium (Palla-
dium, JCPDS 46-1043, 1969), respectively. The broadening in the
XRD pattern is most probably due to the amorphous structure of
RGO as the support material. The high resolution TEM (HRTEM)
image of RGO@Pd catalysts given in Fig. 3b is also well-matched
with the powder XRD pattern, where is only (1 1 1) plane of fcc-Pd
observable. A typical interfringe distance of 0.221 nm was calcu-
lated from the HRTEM image, which is close to the lattice spacing
of the (1 1 1) planes of the face centered cubic crystal of palladium
(0.223 nm) [44].
varied in the range of 10–60 mg (0.2–1.2 mM Pd). In the second
set of experiments, the amount of RGO@Pd was kept constant at
30 mg (0.6 mM Pd) while AB concentration was varied in the range
of 100, 200, 300, 400 and 500 mM. Finally, the hydrolysis of AB
was performed in the presence of the RGO@Pd catalyst at con-
stant AB (200 mM) and catalyst concentration 0.6 mM Pd (30 mg
RGO@Pd) at various temperatures involved a range of 20–40 ◦C in
order to obtain apparent Arrhenius activation energy (Eaapp) and
other activation parameters.
2.7. Durability of the RGO@Pd catalyst in the hydrolytic
dehydrogenation of AB
In a typical reusability test, RGO@Pd catalyst was isolated from
the catalytic reaction solution by filtration after all of the AB present
in solution was consumed for each cycle. The isolated CDG-Pd cat-
alyst was washed several times with water to remove the residuals
from the surface of the catalyst. Next, a new catalytic reaction was
started by dispersing the isolated RGO@Pd catalyst in a solution
containing a new batch of 2.0 mmol AB as described in Section
2.5.
Fig. 2. (a) Low magnification and (b) high magnification of TEM images of RGO@Pd catalyst.

B. Kılıc¸ et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 104–110
107
Fig. 3. (a) Powder-XRD pattern and (b) a representative HR-TEM image of as-prepared RGO@Pd catalyst.
3.2. Kinetic study of the hydrolysis of AB catalyzed by RGO@Pd
catalyst
catalysts or their alloys have been tested in hydrogen generation
from the hydrolysis of AB at room temperature. As clearly under-
stood from Table 1, the TOF value reported here is the best among
all Pd-based catalysts and even comparable to the best Pt-based
catalysts.
Fig. 4a shows the plots of the volume of hydrogen generated
versus time during the catalytic hydrolysis of 200 mM AB solution
in the presence of different amount of RGO@Pd catalysts (10–60 mg
RGO@Pd), which corresponds to concentration in the range of
0.2–1.2 mM Pd at 25 0.5 ◦C. As expected, the hydrogen genera-
tion rate enhanced almost linearly with the increasing amount of
RGO@Pd were used as catalysts in hydrogen generation from
the hydrolysis of AB and found to be highly active. For instance,
the catalytic hydrolysis of 200 mM AB (2 mmol) was completed to
give stoichiometric amount of hydrogen gas (6 mmol H2(g)) within
19 min in the presence of 60 mg RGO@Pd (1.2 mM Pd), which corre-
sponds to TOF of 26.3 min−1. Table 1 shows the activities in terms
of TOF values (mol H2 (mol catalyst)−1 min−1) of the noble metal
Fig. 4. (a) The plots of the volume of hydrogen generated versus time during the
catalytic hydrolysis of 200 mM AB solution in the presence of different Pd concentra-
tions in the range of 0.2–1.2 mM Pd (10–60 mg RGO@Pd catalyst) at 25 0.5 ◦C. (b)
The plot of hydrogen generation rate versus concentration of Pd both in logarithmic
scale.
Fig. 5. (a) The plots of volume of hydrogen generated versus time for catalytic
hydrolysis of AB at different AB concentrations in the range of 100–500 mM of
AB ([Pd] = 0.6 mM; T = 25 0.5 ◦C). (b) The plot of hydrogen generation rate versus
concentration of AB both in logarithmic scale.

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B. Kılıc¸ et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 104–110
Table 1
Activities in terms of TOF values (mol H2 (mol catalyst)−1 min−1) of the noble metal catalysts or their alloys have been tested in hydrogen generation from the hydrolysis of
AB at 25 ◦C. (The TOF values were estimated from the data given in respective references.)
Catalyst
Metal/AB ratio (mol/mol)
Maximum eqv. H2 generated
Completion time (min)
TOF (mol H2 mol catalyst−1 min−1
)
Reference
20 wt% Pt/C
Co35Pd65/C annealed
2.1 wt% RGO@Pd
25 wt% Co35Pd65/C
PtO2
Pt black
K2PtCl4
Pd/zeolite
0.018
0.024
0.006
0.024
0.018
0.018
0.018
0.02
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
6
3.5
18
5.5
8
12
19
24
12
55.4
[13]
[51]
This study
[51]
[13]
[13]
[13]
[24]
[16]
35.7
26.3
22.7
20.8
13.9
8.8
6.25
5
1.39
0.67
PSSA-co-MA-Pd
2 wt% Pd/␥-Al2O3
Pd black
0.02
0.025
0.05
120
250
[22]
[21]
The apparent activation energy was calculated to be (Eaapp) =
40 2 kJ mol−1 by using Arrhenius plot shown in Fig. 7a. The
activation energy of RGO@Pd catalyst is also comparable with
the other noble metal-based catalyst systems reported for the
hydrolysis of AB (see Table 2). Additionally, the other apparent
activation parameters; activation enthalpy (ꢀH#,app) and activa-
tion entropy (ꢀS#,app) were calculated to be 38 1 kJ mol−1 and
−134 1 J K−1 mol−1 from the slope of Eyring plot in Fig. 7b.
The durability of the RGO@Pd catalyst in the hydrolysis of
AB was studied by performing a reusability test. Fig. 8a shows
the mol H2 per mol AB versus time during the reusability
test RGO@Pd (0.6 mM Pd) catalysts in the hydrolysis of AB
(200 mM) at room temperature. The initial TOF of RGO@Pd
catalysts decreased from 930 to 890 after 10th run, which
means that they preserve 95% of their initial activity after 10th
runs. Fig. 8b shows a representative TEM image RGO@Pd cata-
lyst after the ten-run durability test. As clearly seen from the
TEM image, there is no noticeable change in the morphology
RGO@Pd catalyst. The hydrogen generation rate was determined
from the initial linear portion of the plot for each catalyst concen-
tration and used to form Fig. 4b that shows the plot of hydrogen
generation rate versus concentration of Pd both in logarithmic
scale. One obtains a straight line with a slope of 1.12 ≈ 1 in Fig. 4b
indicating that hydrolysis of AB is first order with respect to the Pd
concentration of RGO@Pd catalyst.
The effect of AB concentration on the hydrogen generation rate
was also studied. Fig. 5 summarises the results on a series of
experiments performed at 25 0.5 ◦C with various initial AB con-
centrations and constant catalyst concentration at 0.6 mM Pd. It
can be concluded from the slope of line in Fig. 5b that the hydrogen
generation from the catalytic hydrolysis of AB is practically inde-
pendent of the AB concentration. This indicates that the hydrolysis
reaction is zeroth order with respect to the substrate concentra-
tion. Actually, the independency of AB concentration is plausible
result considering our catalytic reaction conditions in which at least
100-fold excess AB were used compared to catalyst concentration.
Consequently, the rate law for the catalytic hydrolysis of AB can
be given as in Eq. (2);
−3d[NH3BH3]
d[H2]
dt
=
= k[Pd]
(2)
dt
The RGO@Pd catalyzed hydrolysis of AB was carried out at vari-
ous temperatures (25–40 ◦C) in the presence of 200 mM AB (63 mg)
and 0.6 mM Pd (Fig. 6). The values of rate constant k at different
temperatures were calculated from the slope of the linear region
of each plot in Fig. 6 by using rate law equation and used to draw
Arrhenius and Eyring plots (Fig. 7).
Fig. 7. (a) The Arrhenius plot (ln k versus the reciprocal absolute temperature 1/T
(K−1)). (b) The Eyring plot (ln(k/T) versus the reciprocal absolute temperature 1/T
(K−1)).
Fig. 6. The plots of volume of hydrogen generated versus time for catalytic hydrol-
ysis of AB at different temperatures ([AB] = 200 mM; [Pd] = 0.6 mM).

B. Kılıc¸ et al. / Journal of Molecular Catalysis A: Chemical 361–362 (2012) 104–110
109
Table 2
Apparent activation energies (Eaapp, kJ mol−1) for the hydrolysis of ammonia borane in the presence of noble metal-based catalysts.
Catalyst
Apparent activation energy Eaapp (kJ mol−1
)
Reference
Pt/␥-Al2O3
Rh/␥-Al2O3
Ru/␥-Al2O3
2.1 wt% RGO/Pd
PSSA-co-MA stabilized Pd(0) nanoclusters
PSSA-co-MA stabilized Ru(0) nanoclusters
Hydroxyapatite/Pd
Zeolite framework stabilized Pd nanoclusters
Pd-PVB-TiO2
Zeolite framework stabilized Rh nanoclusters
Ru/C
K2PtCl6
21
21
23
40
44
54
55
56
56
67
76
87
[22]
[22]
[22]
This study
[16]
[16]
[46]
[24]
[47]
[49]
[50]
[48]
Fig. 8. (a) The plots of the mol H2/mol AB versus time for RGO@Pd catalyzed hydrolysis of AB at different catalytic runs during the ten-run durability test. (b) A representative
TEM image of RGO@Pd catalyst after ten-run durability test.
provided the best TOF value (26.3 min−1) among all Pd-based cat-
alysts and even comparable to the Pt-base catalysts. Besides its
high activity RGO@Pd catalyst was also proved to be a highly stable
catalyst in the hydrolysis of AB providing 11,600 total turnovers
in 46 h. The first order kinetics with respect to the concentration
and the zeroth order kinetics with respect to the AB concentration
were elucidated for the RGO@Pd catalyzed hydrolysis of AB. These
results indicate that the RGO@Pd is a promising heterogeneous cat-
alyst among other noble metal-based catalyst systems in hydrogen
generation from the AB hydrolysis under ambient conditions.
and particle size of Pd nanoparticles on RGO after catalysis, which
means that they are durable catalyst in the hydrolysis of AB.
Catalyst lifetime measured as total turnover number (TTON) was
evaluated in 10.0 mL aqueous solution of RGO@Pd catalyst (0.4 mM
Pd) and 0.5 M AB at 25.0 0.5 ◦C. The RGO@Pd catalyst provided
TTON of 11,600 over 46 h in the hydrolysis of AB. The ammonia, one
of the possible products generated due to the incomplete hydroly-
sis, was also monitored at certain time intervals by using acid/base
titration test with the detection limit of ∼0.12% by weight [23]. In all
RGO@Pd catalyzed hydrolysis reactions, no ammonia was detected,
indicating the complete hydrolysis of AB complex. Actually, the
observation of no ammonia generation during the hydrolysis of AB
is not surprising in our reaction conditions which the amount of
AB and Pd kept smaller than 2 wt% and 0.05 wt%, respectively [23].
Hydrogen generation slows down as the reaction proceeds because
the viscosity of the reaction solution increased due to the formation
of metaborate, which is the major problem for the deactivation of
the catalyst. Therefore, if the increase in viscosity could be avoided
or metaborate could be removed from the reaction system, a much
higher TTON should be obtained.
Acknowledgment
The financial support by Atatürk University Scientific Research
Project Council (Project No: 2011/93) is gratefully acknowledged.
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In summary, we have demonstrated a facile method for the
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