Hydrogen generation from hydrolysis of ammonia borane in the presence of highly efficient poly(N-vinyl-2-pyrrolidone)-protected platinum-ruthenium nanoparticles

Article abstract of DOI:10.1016/j.apcata.2014.03.022

Herein, the employment of PVP-protected Pt-Ru bimetallic nanoparticles (3.2 ± 1.4 nm) as highly efficient catalysts in the hydrolysis of ammonia borane for hydrogen generation is reported. They were prepared by co-reduction of two metal ions in ethanol/water mixture by an alcohol reduction method and characterized by TEM-EDX analysis, UV-vis spectroscopy, and X-ray photoelectron spectroscopy. They are recyclable and highly active for hydrogen generation from the hydrolysis of ammonia borane even at very low concentrations and temperature, providing a record numbers of average TOF value (308 mol H ₂ mol_cat^-1 min^-1) and maximum hydrogen generation rate (9884 L H₂ min^-1 (mol _cat)^-1) for ammonia borane. PVP-protected Pt-Ru bimetallic nanoparticles provide activation energy of 56.3 ± 2 kJ mol^-1 for the hydrolysis of ammonia borane.

Full text of DOI:10.1016/j.apcata.2014.03.022

Applied Catalysis A: General 478 (2014) 15–20
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Hydrogen generation from hydrolysis of ammonia borane in the
presence of highly efficient poly(N-vinyl-2-pyrrolidone)-protected
platinum-ruthenium nanoparticles
Murat Rakap∗
Department of Chemistry, Yuzuncu Yil University, 65080, Van, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 January 2014
Received in revised form 26 February 2014
Accepted 17 March 2014
Available online 25 March 2014
Herein, the employment of PVP-protected Pt–Ru bimetallic nanoparticles (3.2 1.4 nm) as highly
efficient catalysts in the hydrolysis of ammonia borane for hydrogen generation is reported. They
were prepared by co-reduction of two metal ions in ethanol/water mixture by an alcohol reduction
method and characterized by TEM–EDX analysis, UV–vis spectroscopy, and X-ray photoelectron spec-
troscopy. They are recyclable and highly active for hydrogen generation from the hydrolysis of ammonia
borane even at very low concentrations and temperature, providing a record numbers of average TOF
Keywords:
Platinum
Ruthenium
Nanoparticles
Ammonia borane
Hydrogen
−1
value (308 mol H2 molcat−1 min−1) and maximum hydrogen generation rate (9884 L H2 min−1 (molcat
)
)
for ammonia borane. PVP-protected Pt–Ru bimetallic nanoparticles provide activation energy of
56.3 2 kJ mol−1 for the hydrolysis of ammonia borane.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
can liberate hydrogen upon hydrolysis at room temperature in the
presence of suitable catalysts according to Eq. (1) [11].
Hydrogen has been considered as a clean and environmentally
benign new energy carrier for heating, transportation, mechanical
power and electricity generation [1,2], since there is an increasing
demand for the renewable energy sources due to the depletion of
fossil fuel resources, environmental pollution, and global warming
caused by a steep increase in concentration of carbon dioxide and
other greenhouse gases in the atmosphere [3–6]. However, the
storage of hydrogen is still a challenge for the implementation of
hydrogen economy [7]. Among the chemical hydrides tested as
solid hydrogen storage materials, ammonia borane (H3NBH3, AB)
appears to be suitable hydrogen source due to a number of advan-
tageous properties such as the high hydrogen storage capacity
(19.6% wt) which meets the US-DOE criteria for hydrogen storage
materials [3,8], the optimal control on hydrogen generation rate
by supported catalysts, the acceptable hydrogen generation rate
even at low temperature, high solubility in water, the availability
and easy handling [9,10]. Additionally, it has an advantage of the
stability of aqueous solutions to self-hydrolysis, and there is no
need to add any base to stabilize the aqueous solutions of AB. AB
H3NBH3(aq) + 2H2O(L)ca−ta→lystNH+(aq) + BO−(aq) + 3H2(g)
(1)
4
2
Various catalysts including Ni1−xPtx hollow spheres [12],
Cu@Cu2O core shell catalysts [13], hollow Ni–SiO2 nanosphere
[14], Fe(0) NPs [15], PVP-protected Ni(0) [16] and Co(0) NCs
[17], polymer-stabilized Ru(0) and Pd(0) NCs [18], magnetically
recyclable Fe–Ni alloy [19], Co(0) NPs [20], water/air-stable Ni
NPs [21], Co–Mo–B/Ni foam [22], hollow Co–B nanospindles [23],
nanoparticle-assembled Co–B thin film [24], bimetallic Au–Ni NPs
embedded in SiO2 nanospheres [25], Cu/Co3O4 NPs [26], elec-
troless Co–Ni–P/Pd–TiO2 [27], Pd–PVB–TiO2 [28], SiO2 supported
monodisperse Ni NPs [29], Co–SiO2 nanosphere [30], graphene
oxide supported Pd NPs [31], and Ru NPs [32] have been tested
for the hydrolysis of AB.
Among these catalysts, nanoparticle-types provided good cat-
alytic activities as expected, due to the small particle sizes.
However, the addition of second element to the monometal-
lic nanoparticles will definitely improve the catalytic properties
[33,34]. Therefore, it has been focused on the employment of highly
effective bimetallic-type nanoparticles as catalyst for hydrogen
generation from the hydrolysis of AB, recently. To the knowledge,
there is no reported work on the catalytic activity of poly(N-vinyl-
2-pyrrolidone)-protected platinum–ruthenium nanoparticles in
the hydrogen generation from the hydrolysis of AB. Herein, the
∗
Tel.: +90 432 225 10 25.
E-mail addresses: mrtrakap@gmail.com, muratrakap@gmail.com
http://dx.doi.org/10.1016/j.apcata.2014.03.022
0926-860X/© 2014 Elsevier B.V. All rights reserved.

16
M. Rakap / Applied Catalysis A: General 478 (2014) 15–20
employment of highly efficient PVP-protected Pt–Ru bimetallic
nanoparticles (will be referred as Pt–Ru@PVP nanoparticles here-
after) for hydrogen generation from the hydrolysis of AB is reported.
They were prepared by an alcohol reduction method [35] and
characterized by TEM–EDX analysis, UV–vis spectroscopy, and X-
ray photoelectron spectroscopy. Additionally, the formation of
PVP-protected Pt–Ru nanoparticles was confirmed by comparing
the catalytic activities of monometallic platinum and ruthenium
nanoparticles and the physical mixture of them with the cat-
alytic activity of bimetallic nanoparticles.The kinetic studies were
carried out depending on the catalyst/substrate concentrations
and temperature. Although the cost of noble metal catalysts is
assumed to be high, the high catalytic activity and effectivity of
the PVP-protected Pt–Ru nanoparticles make them a very promis-
ing candidate to be used as catalyst in developing efficient portable
hydrogen generation systems using AB as solid hydrogen storage
material since it would compensate the cost concerns.
hemispherical analyzer and using monochromatic Mg-K␣ radiation
(1250 eV, the X-ray tube working at 15 kV and 350 W).
2.3.4. 11B NMR spectra
11B NMR spectra were recorded on a Bruker Avance DPX 400
with an operating frequency of 128.15 MHz for 11B. D2O and
BF3·(C2H5)2O were used as a lock and external reference, respec-
tively. At the end of the hydrolysis reaction, the resulting solutions
were filtered and the filtrates used for taking 11B NMR spectra.
2.4. Method to test the catalytic activity of Pt–Ru@PVP
nanoparticles in the hydrolysis of AB
The catalytic activity of Pt–Ru@PVP nanoparticles in the hydrol-
ysis of AB in aqueous solution was determined by measuring the
rate of hydrogen generation. In all the experiments, a jacketed reac-
tion flask (50 mL) containing a Teflon-coated stir bar was placed
on a magnetic stirrer (Heidolph MR-301) and thermostated to
25.0 0.1 ◦C by circulating water through its jacket from a constant
temperature bath. Then, a graduated glass tube (40 cm in height and
2.5 cm in diameter) filled with water was connected to the reaction
flask to measure the volume of the hydrogen gas to be evolved
from the reaction. In a typical experiment, 63.6 mg (2 mmol) of
H3NBH3 was dissolved in 20 mL of water. The solutions were trans-
ferred with a glass pipet into the reaction flask thermostated at
25.0 0.1 ◦C. Then, aliquots of Pt–Ru@PVP nanoparticles from the
stock solution (5.0 mM) were added into the reaction flask. The
experiment was started by closing the flask and the volume of
hydrogen gas evolved was measured by recording the displacement
of water level at the stirring speed of 900 rpm. In addition to the
volumetric measurement of the hydrogen evolution, the conver-
sion of AB (ı = −23.9 ppm) [11] to metaborate (ı = 9 ppm) [36] was
also checked by 11B NMR spectroscopy.
2. Experimental
2.1. Materials
Ruthenium(III) chloride trihydrate (RuCl3·3H2O), hexachloro-
platinic(IV) acid hexahydrate (H2PtCl6·6H2O), poly(N-vinyl-2-
pyrrolidone) (PVP-40), and ammonia borane (H3NBH3) were
purchased from Aldrich. Ethanol was purchased from Merck.
Deionized water was distilled by a water purification system (Milli-
Q system). All glassware and Teflon-coated magnetic stir bars were
cleaned with acetone, followed by copius rinsing with distilled
water before drying in an oven at 150 ◦C.
2.2. The preparation of PVP-protected Pt–Ru@PVP nanoparticles
Pt–Ru@PVP nanoparticles were prepared by an alcohol reduc-
tion method. First, solutions of ruthenium(III) chloride trihydrate
(0.25 mmol in 25 mL ethanol) and hexachloroplatinic(IV) acid
hexahydrate (0.25 mmol in 25 mL water) were mixed and poly(N-
vinyl-2-pyrrolidone) (PVP-40, 2.5 mmol of monomeric units) was
added to this solution as a protecting polymer. Then, the mixed
solution was refluxed at 90 ◦C for 2 h. The formed Pt–Ru nanopar-
ticles have brownish black color and stable for months at room
temperature. The total concentration of both metals was kept as
5.0 mM in 50 mL of the mixed solution.
2.5. Determination of activation energy of Pt–Ru@PVP
nanoparticles in the hydrolysis of AB
In a typical experiment, the hydrolysis of AB (100 mM) catalyzed
by Pt–Ru@PVP nanoparticles (0.3 mM) was performed by following
the same procedure described in Section 2.4 at various temper-
atures (10, 15, 20, 25, and 30 ◦C) to obtain the activation energy
(Ea).
2.6. Recyclability of Pt–Ru@PVP nanoparticles in the hydrolysis of
AB
2.3. The characterization of Pt–Ru@PVP nanoparticles
The recyclability of Pt–Ru@PVP nanoparticles in the hydrol-
ysis of AB was determined by a series of experiments started
with a 20 mL solution containing 0.3 mM Pt–Ru@PVP nanoparticles
and 0.100 M AB at 25.0 0.1 ◦C. When the complete conversion is
achieved, another equivalent of AB was immediately added to the
reaction mixture. The results were expressed as % initial catalytic
activity of Pt–Ru@PVP nanoparticles versus the number of catalytic
runs in the hydrolysis of AB solution.
2.3.1. UV–vis analysis
UV–vis spectra were recorded on a Cary 5000 (Varian) UV–vis
spectrophotometer. A quartz cell with a part length of 1 cm was
used and spectra were collected over the range of 200–900 nm.
2.3.2. TEM–EDX analysis
Transmission electron microscopy (TEM) analysis was carried
out using a JEOL-2010 microscope operating at 200 kV, fitted with
a LaB6 filament and has lattice and theoretical point resolutions of
0.14 nm and 0.23 nm, respectively. Samples were examined at mag-
nification between 100 and 400 K. One drop of dilute suspension of
sample was deposited on the TEM grids and the solvent was then
evaporated. The diameter of each particle was determined from the
enlarged photographs.
3. Results and discussion
3.1. Preparation and characterization of PVP-protected Pt–Ru
nanoparticles
Pt–Ru@PVP nanoparticles were prepared from the co-reduction
of mixture of hexachloroplatinic(IV) acid hexahydrate and ruthe-
nium(III) chloride trihydrate by an alcohol reduction method in the
presence of PVP in ethanol–water mixture at refluxing tempera-
ture. PVP serves as stabilizer and reducing agent. After refluxing
2.3.3. X-Ray photoelectron spectroscopy
X-ray photoelectron spectrum (XPS) of the isolated nanopar-
ticles was taken by using SPECS spectrometer equipped with a

M. Rakap / Applied Catalysis A: General 478 (2014) 15–20
17
for 2 h, the color of the solution turned to brownish black, indi-
cating the reductions of Pt4+ and Ru3+ ions to Pt0 and Ru0 to form
nanoparticles of them. Monitoring the UV–vis electronic absorption
spectra of the solution provides the best way to follow this con-
version. Fig. 1 shows the spectral change during the formation of
Pt–Ru@PVP nanoparticles from the reduction corresponding plat-
inum and ruthenium salts by PVP. The absorption bands due to
d–d transitions in Pt4+ and Ru3+ ions completely disappear after
refluxing the solution, indicating the complete reduction of the
corresponding ions. The catalyst was found to be highly stable for
weeks with any precipitation of the metal particles at room tem-
perature.
The size, morphology and composition of Pt–Ru@PVP nanopar-
ticles were investigated by TEM–EDX analyses. Fig. 2 shows the TEM
images taken at different magnifications (a—50, b—20, c—2 nm,
and d—after fifth hydrolysis run) and the EDX spectrum (Fig. 2e)
of Pt–Ru@PVP nanoparticles, confirming alloy structure and 1:1
ratio of Pt:Ru. 1:1 ratio of Pt:Ru was also confirmed by ICP anal-
ysis. The mean particle size was determined as 3.2 1.4 nm from
TEM images by counting 226 non-touching particles.
Fig. 1. UV–vis absorption spectra of the aqueous solutions of H2PtCl6·6H2O,
RuCl3·3H2O, and Pt–Ru@PVP nanoparticles.
Fig. 2. TEM images ((a) 50, (b) 20 nm, (c) 2 nm, and (d) after fifth hydrolysis run) and the EDX spectrum (e) of Pt–Ru@PVP nanoparticles.

18
M. Rakap / Applied Catalysis A: General 478 (2014) 15–20
Fig. 3. X-Ray photoelectron spectrum of Pt–Ru@PVP nanoparticles.
Fig. 5. Plot of mol H2 mol−1 AB versus time for the hydrolysis of 0.100 M AB solutions
in the presence of Pt–Ru@PVP nanoparticles in different catalyst concentrations (0.1,
0.2, 0.3, 0.4, and 0.5 mM) at 25.0 0.1 ◦C.
The PVP-protected Pt–Ru@PVP nanoparticles were isolated by
evaporating all the solvent. The isolated samples of the catalyst
were used for the XPS analysis of the catalyst surface. The main
peaks observed in the survey scan XPS (Fig. 3) are C 1s, Pt 4d, Pt 4f,
Ru 3d, Ru 3p, and O 1s located at 285, 320–350, 65–80, 280–285,
460–480, and 530 eV, respectively. The XPS spectra for Pt 4f are
characterized by a doublet containing a binding energy of 70.3 eV
for 4f7/2 and 73.8 eV for 4f5/2, confirming the presence Pt(0) [37].
Due to the overlap of the C 1s and Ru 3d peaks around 285 eV, it
is very difficult to analyze this region for ruthenium properly. The
peak located at 462 eV for Ru 3p3/2 is readily assigned to the Ru(0)
[38]. There is no higher oxidation state peaks for both metals of the
catalyst in the XPS spectra, indicating the protection of Pt(0) and
Ru(0) species by the attachment of PVP during catalyst preparation
procedure.
Additionally, the formation of PVP-protected Pt–Ru nanoparti-
cles rather than the physical mixtures of individual monometallic
nanoparticles was confirmed by comparing the catalytic activities
of all the types (monometallic Pt and Ru nanoparticles, 1:1 phys-
ical mixture of them, and 1:1 Pt–Ru bimetallic nanoparticles) in
the hydrolysis of AB. As shown in Fig. 4, Pt–Ru@PVP nanoparticles
provided a much higher catalytic activity than the physical mix-
ture of Pt and Ru monometallic nanoparticles, clearly indicating
that the prepared catalyst is composed of Pt–Ru@PVP bimetallic
nanoparticles rather than a mixture of the individual monometal-
lic nanoparticles. This much higher catalytic activity of bimetallic
nanoparticles stems from the synergistic effects of platinum and
ruthenium [33] and the reduced particles size compared to the indi-
vidual platinum (4.6 nm) and ruthenium (4.2 nm) nanoparticles.
3.2. Testing the catalytic activity of Pt–Ru@PVP nanoparticles in
the hydrolysis of AB
Pt–Ru@PVP nanoparticles were found to be highly efficient cat-
alyst for the hydrolysis of AB. Fig. 5 shows the plots of the volume
of generated hydrogen gas versus time (plot of mol H2 mol−1 AB
versus time) in the catalytic hydrolysis of 0.100 M AB solutions in
the presence of Pt–Ru@PVP nanoparticles in different catalyst con-
centrations at 25.0 0.1 ◦C. The linear hydrogen generation starts
immediately without an induction period and continues until the
complete hydrolysis of AB. The quantity of ammonia liberated dur-
ing the hydrolysis of AB has been found to be negligible when the
catalyst and substrate concentrations are lower than 0.06 mol% and
6 wt%, respectively [39]. As expected, the control tests using cop-
per(II) sulfate trap with acid–base indicators resulted in no NH3
evolution in detectable amount in the experiments conducted in
this study. Additionally, conversion of AB (ı = −23.9 ppm) to metab-
orate(ı = 9 ppm)was also checkedby 11BNMR spectroscopy(Fig. 6).
The signal of external reference, BF3·(C2H5)2O, at 0 ppm is not
shown in the figure.
Fig. 7 shows the plots of the volume of generated hydrogen
gas versus time in the catalytic hydrolysis of 0.100 M AB solu-
tions in the presence of Pt–Ru@PVP nanoparticles (0.3 mM) at
various temperatures (in the range of 10–30 ◦C). It is worth to
note that using Pt–Ru@PVP nanoparticles (0.3 mM) leads to com-
plete hydrogen release (3.0 mol H2 mol−1 AB) for the hydrolysis
of AB within 195 s, corresponding to record average TOF value
of 308 mol H2 molcat−1 min−1 an−d1maximum hydrogen generation
rate of 9884 L H2 min−1 (molcat
)
at 25.0 0.1 ◦C. The TOF values
of various catalysts in the hydrolysis of AB are given in Table 1, for
comparison.
The rate constants of hydrogen generation from the hydrolysis
of AB were calculated from the linear portions of the plots given
in Fig. 7 at five different temperatures and used for the calculation
of the activation energy (Ea = 56.3 2 kJ mol−1 for the hydrolysis of
AB) from the Arrhenius plot (Fig. 8) for hydrolysis reaction. This
activation energy of 56.3 2 kJ mol−1 for the hydrolysis of AB is
Fig. 4. Comparison of the catalytic activities of 0.3 mM of Pt and Ru monometallic
nanoparticles, physical mixture of them, and Pt–Ru@PVP bimetallic nanoparticles
in the hydrolysis of 0.100 M AB at 25.0 0.1 ◦C.

M. Rakap / Applied Catalysis A: General 478 (2014) 15–20
19
Table 1
Activities in terms of TOF values of various catalyst systems tested in hydrogen
generation from the hydrolysis of AB (The TOF values were either directly taken or
estimated from the data given in respective references.).
Catalyst
TOF (mol H2 mol
catalyst−1 min−1
Reference
)
Pt–Ru@PVP NPs
Laurate-stabilized Rh(0) NCs
Zeolite-Rh(0) NCs
Laurate-stabilized Ru(0) NCs
Pt/C
308.0
200.0
92.0
75.0
55.4
44.2
39.8
39.6
35.7
26.3
25.7
20.8
15.5
13.9
13.7
10.9
10.1
10.0
This study
[46]
[47]
[48]
[49]
[20]
[16]
[32]
[50]
[31]
[44]
[49]
[51]
[49]
[52]
[21]
[44]
[36]
Co
In-situ Co(0) NPs
Ru@Al2O3 NPs
Co35Pd65/C
RGO-Pd NPs
PSSA-co-MA stabilized Co(0) NCs
PtO2
Graphene-Pd(0) NPs
Pt black
Au@Co core–shell NPs
In-situ Fe1−xNix NPs
PSSA-co-MA stabilized Ni(0) NCs
Electroplated Co–P
Fig. 6. 11B NMR spectrum (a) before and (b) after hydrolysis of 0.100 M AB catalyzed
by 0.3 mM Pt–Ru@PVP nanoparticles.
lower than the activation energies reported in the literature for the
same reaction using many different catalysts: 70 kJ mol−1 for bulk
nickel [40], 63 kJ mol−1 for PVP-stabilized cobalt(0) nanoclusters
[17], but still higher than 56 kJ mol−1 for zeolite confined palla-
dium(0) nanoclusters [41], 52 kJ mol−1 for Ni–Ag/C [42], 39 kJ mol−1
for Pt0.65Ni0.35 nanoparticles [43], 44 kJ mol−1 for PSSA-co-MA
stabilized Pd(0) nanoclusters [44], 44 kJ mol−1 for (Co–Mo–B)/Ni
foam [22], and 34 kJ mol−1 for SiO2-supported monodisperse nickel
nanoparticles [29].
3.3. Recyclability of Pt–Ru@PVP nanoparticles in the hydrolysis of
AB
The recyclability of Pt–Ru@PVP nanoparticles in the hydroly-
sis of AB was investigated by successive additions of AB after the
first cycle of the hydrolysis reaction. Fig. 9 shows the results of the
recyclability tests for the hydrolysis of AB catalyzed by Pt–Ru@PVP
nanoparticles. The Pt–Ru@PVP nanoparticles catalyst retains 72%
of its initial catalytic activity in the hydrolysis of AB, even at
the fifth run. The decrease in the catalytic activity of Pt–Ru@PVP
nanoparticles in the hydrolysis of AB is due to the passivation of
nanoparticles’ surface by increasing amount of metaborate, which
Fig. 7. Plots of the volume of generated hydrogen gas versus time in the catalytic
hydrolysis of 0.100 M AB solutions in the presence of Pt–Ru@PVP nanoparticles
(0.3 mM) at various temperatures (10, 15, 20, 25, and 30 ◦C).
Fig. 8. Arrhenius plot for the hydrolysis of AB (0.100 M) catalyzed by 0.3 mM
Pt–Ru@PVP nanoparticles.
Fig. 9. Retained % catalytic activity of 0.3 mM Pt–Ru@PVP nanoparticles in the suc-
cessive catalytic runs for the hydrolysis of 0.100 M AB at 25.0 0.1 ◦C.

20
M. Rakap / Applied Catalysis A: General 478 (2014) 15–20
decreases accessibility of active sites [45] and the aggregation of
nanoparticles as shown in the TEM image of the catalyst taken after
fifth run of the hydrolysis reaction (Fig. 2d).
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•
Pt–Ru@PVP nanoparticles can be easily prepared from the co-
reduction of corresponding platinum and ruthenium salts by an
alcohol reduction method.
•
Pt–Ru@PVP nanoparticles are highly efficient catalyst for hydro-
gen generation from the hydrolysis of AB.
•
They provide
(308 mol H2 molcat−1 min−1) and maximum hydrogen generation
rate (9884 L H2 min−1 (molcat −1) for the hydrolysis of AB.
a record numbers of average TOF value
)
[25] H.L. Jiang, T. Umegaki, T. Akita, X.B. Zhang, M. Haruta, Q. Xu, Chem. Eur. J. 16
(2010) 3132–3137.
•
•
Activation energy for the catalytic hydrolysis of AB in
the presence of Pt–Ru@PVP nanoparticles was calculated as
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16456–16462.
C 114 (2010)
56.3 2 kJ mol−1
.
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Pt–Ru@PVP nanoparticles can be regarded as promising catalysts
having high activity for practical applications to supply hydrogen
from the hydrolysis of AB for proton exchange membrane fuel
cells.
Acknowledgment
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oratory of Middle East Technical University.
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