PVP-stabilized Ru-Rh nanoparticles as highly efficient catalysts for hydrogen generation from hydrolysis of ammonia borane

Article abstract of DOI:10.1016/j.jallcom.2015.07.249

Abstract Herein, the utilization of poly(N-vinyl-2-pyrrolidone)-protected ruthenium-rhodium nanoparticles (3.4 ± 1.4 nm) as highly efficient catalysts in the hydrolysis of ammonia borane for hydrogen generation is reported. They are prepared by co-reduction of ruthenium and rhodium metal ions in ethanol/water mixture by an alcohol reduction method and characterized by transmission electron microscopy-energy dispersive X-ray spectroscopy, ultraviolet-visible spectroscopy, and X-ray photoelectron spectroscopy. They are durable and highly efficient catalysts for hydrogen generation from the hydrolysis of ammonia borane even at very low concentrations and temperature, providing average turnover frequency of 386 mol H₂ (mol cat)^-1 min^-1 and maximum hydrogen generation rate of 10,680 L H₂ min^-1 (mol cat)^-1. Poly(N-vinyl-2-pyrrolidone)-protected ruthenium-rhodium nanoparticles also provide activation energy of 47.4 ± 2.1 kJ/mol for the hydrolysis of ammonia borane.

Full text of DOI:10.1016/j.jallcom.2015.07.249

Journal of Alloys and Compounds 649 (2015) 1025e1030
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
PVP-stabilized RueRh nanoparticles as highly efficient catalysts for
hydrogen generation from hydrolysis of ammonia borane
Murat Rakap
Maritime Faculty, 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:
Herein, the utilization of poly(N-vinyl-2-pyrrolidone)-protected rutheniumerhodium nanoparticles
(3.4 1.4 nm) as highly efficient catalysts in the hydrolysis of ammonia borane for hydrogen generation
is reported. They are prepared by co-reduction of ruthenium and rhodium metal ions in ethanol/water
mixture by an alcohol reduction method and characterized by transmission electron microscopy-energy
dispersive X-ray spectroscopy, ultravioletevisible spectroscopy, and X-ray photoelectron spectroscopy.
They are durable and highly efficient catalysts for hydrogen generation from the hydrolysis of ammonia
borane even at very low concentrations and temperature, providing average turnover frequency of
Received 20 May 2015
Received in revised form
23 July 2015
Accepted 27 July 2015
Available online 29 July 2015
Keywords:
Ruthenium
Rhodium
Nanoparticle
Ammonia borane
Hydrogen
386 mol H2 (mol cat)ꢀ1 minꢀ1 and maximum hydrogen generation rate of 10,680 L H2 minꢀ1 (mol cat)ꢀ1
.
Poly(N-vinyl-2-pyrrolidone)-protected rutheniumerhodium nanoparticles also provide activation en-
ergy of 47.4 2.1 kJ/mol for the hydrolysis of ammonia borane.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
(M ¼ Fe, Co, Ni) nanoparticles [15], AuCo@MOF nanoparticles
[16], PteCo@GO nanoparticles [17], NiPd@rGO nanoparticles [18],
Employment of chemical hydrides as efficient and safe hydrogen
storage materials is one of the most promising approaches [1]. For
this purpose, ammonia borane (H3NBH3, AB) has recently been
employed as a solid hydrogen storage material due to its high
hydrogen content (19.6 wt %), high solubility and stability in water
at room temperature [2,3]. AB is able to release hydrogen upon
hydrolysis at room temperature in the presence of suitable catalysts
according to Eq. (1):
CuNi nanoparticles [19], and Cu@Co on rGO nanoparticles [20] are
the examples of those type of catalysts. Very recently, poly(N-vinyl-
2-pyrrolidone (PVP))-protected bimetallic nanoparticles, such as
rutheniumepalladium [21], platinumeruthenium [22], palla-
diumerhodium [23], and palladiumeplatinum [24] nanoparticles,
have been shown to be highly efficient catalysts for hydrogen
generation from boron compounds providing remarkable results.
This study reports the first time use of PVP-protected ruth-
eniumerhodium nanoparticles (RueRh@PVP NPs) as highly effi-
cient catalysts for the hydrolysis of AB as a complementary study
for the aforementioned catalysts. The catalysts were prepared by a
modified alcohol reduction method [25], found to be stable as
colloidal dispersions, and characterized by transmission electron
microscopy-energy dispersive X-ray spectroscopy (TEM-EDX),
ultravioletevisible spectroscopy (UVeVis), and X-ray photoelec-
tron spectroscopy (XPS). Although the cost of noble metal catalysts
is assumed to be high, the high catalytic activity of the RueRh@PVP
nanoparticles makes them very promising candidates to be used as
catalyst in developing efficient portable hydrogen generation sys-
tems using AB as solid hydrogen storage material since it would
easily compensate the cost concerns.
H NBH ðaqÞ þ 2H OðlÞcatalystNHþðaqÞ þ BOꢀðaqÞ þ 3H ðgÞ
ƒƒƒ!
3
3
2
2
2
4
(1)
Recently, different kinds of bimetallic nanoparticle-type cata-
lysts have been tested for hydrogen generation from the hydrolysis
of AB since the addition of second element to the monometallic
nanoparticles definitely improves the catalytic properties. Ni@Ru
coreeshell nanoparticles [4], NieRu alloy nanoparticles [5], RuCo
and RuCu on ɤ-Al2O3 [6], RuCu on graphene [7], Ru@Ni coreeshell
nanoparticles [8], CueNi on MCM-41 [9], Ru@Co on graphene [10],
CoNi@rGO [11], CuCo@MOF nanoparticles [12], Ni/Pt hollow
nanospheres [13], Ag/Pd@nanofiber nanoparticles [14], Pt-M
E-mail address: mrtrakap@gmail.com.
http://dx.doi.org/10.1016/j.jallcom.2015.07.249
0925-8388/© 2015 Elsevier B.V. All rights reserved.

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M. Rakap / Journal of Alloys and Compounds 649 (2015) 1025e1030
2. Experimental section
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 solution
was transferred with a glass pipet into the reaction flask thermo-
stated at 298 K. Then, aliquots of RueRh@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 1000 rpm. In addition to the
volumetric measurement of the hydrogen evolution, the conver-
2.1. Materials
Ruthenium(III) chloride trihydrate (RuCl.33H2O), rhodium(III)
chloride trihydrate (RhCl.33H2O), 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 water purification system (Milli Q-pure WS). All glass-
ware and teflon coated magnetic stir bars were rinsed with acetone,
followed by copious washing with distilled water before drying in
an oven at 150 ꢁC.
sion of AB (
d
¼ ꢀ23.9 ppm) [26] to metaborate ( ¼ 9 ppm) [27] was
d
also checked by 11B NMR spectroscopy.
2.2. Preparation of RueRh@PVP nanoparticles
2.5. The effect of stirring speed on hydrogen generation rate
RueRh@PVP nanoparticles were prepared by a modified alcohol
reduction method in which PVP serves as both stabilizer and
reducing agent. First, solutions of ruthenium (III) chloride trihy-
drate (0.25 mmol in 25 mL ethanol) and rhodium (III) chloride
trihydrate (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. Then, the mixed solution was refluxed at
363 K for 2 h RueRh@PVP nanoparticles are brownish black in color
and stable at room temperature. The total concentration of both
metals was kept as 5.0 mM in 50 mL of the mixed solution.
The same experiments described in the Section 2.4 for the
hydrogen generation from the hydrolysis of AB were performed at
298 K by varying the stirring speed (0, 200, 400, 600, 800,1000, and
1200 rpm) to check how hydrogen generation rate from the hy-
drolysis of AB system was affected by stirring speed. The hydrogen
generation rate was found to be independent of the stirring speed
when it is higher than 800 rpm. This indicates that the system is in
a non-mass transfer limitation regime since the present kinetic
study was performed at the stirring speed of 1000 rpm.
2.3. Characterization of RueRh@PVP nanoparticles
2.6. The effect of PVP concentration on the catalytic activity of
RueRh@PVP nanoparticles in the hydrolysis of AB
2.3.1. UVeVis analysis
UVeVis spectra were recorded on a Cary 5000 (Varian) UVeVis
spectrophotometer. A quartz cell with a part length of 1 cm was
used and spectra were collected over the range of 200e900 nm.
In order to study the effect of PVP concentration on the catalytic
activity of RueRh@PVP nanoparticles in the hydrolysis of AB
(100 mM), hydrolysis reactions were carried out in the presence of
catalysts prepared with different [PVP/Cat.] ratios (2, 6, 10, and 14).
All the experiments were conducted in the same way described in
Section 2.4. The optimum [PVP/Cat.] ratio was found to be 10. When
this ratio is lower than 10, the catalytic activity of the catalyst is
relatively low because PVP molecules do not cover the surface of
nanoparticles effectively, leading to decreased catalytic activity by
not preventing the agglomeration of nanoparticles. When it is
higher than 10, catalytic activity of the catalyst starts to decrease
since the surface of the nanoparticles may be wholly covered by
PVP, blocking the active sites to be reached by substrate molecules.
Therefore, optimum [PVP/Cat.] ratio was determined as 10 for
further kinetic studies.
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
magnification between 100 and 400 K. One drop of dilute sus-
pension 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.
2.3.3. X-ray photoelectron spectroscopy
X-ray photoelectron spectrum (XPS) of the isolated nano-
particles was taken by using SPECS spectrometer equipped with a
2.7. Determination of the activation energy of RueRh@PVP
nanoparticles in the hydrolysis of AB
hemispherical analyzer and using monochromatic Mg-K
a radiation
(1250 eV, the X-ray tube working at 15 kV and 350 W).
In a typical experiment, the hydrolysis of AB (100 mM) catalyzed
by RueRh@PVP nanoparticles (0.3 mM) was performed by
following the same procedure described in Section 2.4 at various
temperatures (283, 288, 293, 298, and 303 K) to obtain the acti-
vation energy (Ea).
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. At the end of the
hydrolysis reaction, the resulting solutions were filtered and the
filtrates used for taking 11B NMR spectra.
2.4. Catalytic evaluation of RueRh@PVP nanoparticles in the
2.8. Durability of RueRh@PVP nanoparticles in the hydrolysis of AB
hydrolysis of AB
The recyclability of RueRh@PVP nanoparticles in the hydrolysis
of AB was determined by a series of experiments started with a
20 mL solution containing 0.3 mM RueRh@PVP nanoparticles and
100 mM AB at 298 K. When the complete conversion is achieved,
another equivalent of AB was added to the reaction mixture
immediately. The results were expressed as % initial catalytic ac-
tivity of RueRh@PVP nanoparticles versus the number of catalytic
runs in the hydrolysis of AB solution.
The catalytic activity of RueRh@PVP nanoparticles in the hy-
drolysis of AB in aqueous solution was determined by measuring
the rate of hydrogen generation. In all experiments, a jacketed re-
action flask (50 mL) containing a Teflon-coated stir bar was placed
on a magnetic stirrer and thermostated to 298 K by circulating
water through its jacket from a constant temperature bath. Then, a
graduated glass tube filled with water was connected to the

M. Rakap / Journal of Alloys and Compounds 649 (2015) 1025e1030
1027
3. Results and discussion
3.1. Preparation and characterization of RueRh@PVP nanoparticles
RueRh@PVP nanoparticles were prepared from the co-
reduction of the mixture of ruthenium (III) chloride trihydrate
and rhodium (III) chloride trihydrate by an alcohol reduction
method in the presence of PVP in ethanol-water mixture at
refluxing temperature. PVP serves as stabilizer and reducing agent.
After refluxing for 2 h, the color of the solution turned to brownish
black, indicating reduction of Ru3þ and Rh3þ ions to Ru0 and Rh0 to
form bimetallic nanoparticles of them. Monitoring the UVeVis
electronic absorption spectra of the solution provides the best way
to follow this conversion. Fig. 1 shows the spectral change during
the formation of RueRh nanoparticles from the reduction of cor-
responding ruthenium and rhodium salts by PVP. The absorption
bands due to ded transitions in Ru3þ and Rh3þ ions completely
disappear after refluxing the solution, indicating the complete
reduction of corresponding ions.
Fig. 1. UVeVis absorption spectra of the aqueous solutions of RuCl3$3H2O,
RhCl3$3H2O, and RueRh@PVP nanoparticles.
Fig. 2. TEM images taken at different magnifications (a- 20 nm, b- 2 nm, d- 5 nm after 5th use in hydrolysis reaction) and EDX spectrum (c) of RueRh@PVP nanoparticles.

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M. Rakap / Journal of Alloys and Compounds 649 (2015) 1025e1030
The size, morphology and composition of RueRh@PVP nano-
3.2. Catalytic evaluation of RueRh@PVP nanoparticles in the
hydrolysis of AB
particles were investigated by TEM-EDX analyses. Fig. 2 shows the
TEM images taken at different magnifications (20 nm for Fig. 2a and
2 nm for Fig. 2b) and EDX spectrum (Fig. 2c) of RueRh@PVP
nanoparticles, confirming 1:1 alloy structure which was also
confirmed by ICP analysis. Alloy structure of the catalyst was
further confirmed by measuring the lattice fringe length (Fig. 2b).
The lattice fringe length of RueRh@PVP catalyst is 0.220 nm, which
differs from both the Ru(101) crystal plane (0.206 nm) and the
Rh(100) crystal plane (0.232 nm), showing the crystalline nature of
RueRh alloy nanoparticles. The mean particle size was determined
as 3.4 2.0 nm from TEM image by counting 204 non-touching
particles.
RueRh@PVP nanoparticles were found to be highly efficient
catalysts for the hydrolysis of AB. Fig. 4 shows the plots of mol (H2/
AB) versus time in the catalytic hydrolysis of 100 mM AB solutions
in the presence of RueRh@PVP nanoparticles in different catalyst
The PVP-protected RueRh 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
absorptions observed in the survey scan XPS (Fig. 3) belong to C
1s, Ru 3d, Ru 3p, Rh 3d, O 1s, and N 1s. The XPS spectrum for Rh
3d is characterized by a doublet containing a binding energy of
308.1 eV for 3d5/2 and 313.2 eV for 3d3/2, confirming the presence
of Rh(0) [28]. Due to the strong overlap of the C1s and Ru 3d
peaks around 285 eV, it is very difficult to analyze this region for
ruthenium properly. The absorptions located at 462 eV and
485 eV for Ru 3p3/2 and Ru 3p1/2, respectively, are readily assigned
to the Ru(0) [29]. There were some shifts to the higher binding
energies due to the interaction between ruthenium and rhodium.
There is no higher oxidation state peaks for both metals of the
catalyst in the XPS spectra, indicating the protection of Ru(0) and
Rh(0) species by the attachment of PVP during catalyst prepara-
tion procedure.
Fig. 4. Plot of mol (H2/AB) versus time for the hydrolysis of 100 mM AB solutions in the
presence of RueRh@PVP nanoparticles at different catalyst concentrations (0.1, 0.2, 0.3,
0.4, and 0.5 mM) at 298 K.
Fig. 3. Survey scan X-ray photoelectron spectrum of RueRh@PVP nanoparticles (Fig. 3a) with high resolution spectrum of Ru 3p (Fig. 3b) and (c) Rh 3d (Fig. 3c) regions.

M. Rakap / Journal of Alloys and Compounds 649 (2015) 1025e1030
1029
concentrations (0.1, 0.2, 0.3, 0.4, and 0.5 mM) at 298 K. The linear
hydrogen generation starts immediately without an induction
period and continues until the complete hydrolysis of AB.
the activation energies reported in the literature for the same re-
action using many different catalysts: 52 kJ/mol for RuCu NPs [6],
51.6 kJ/mol for PteCo NPs [17], 51.3 kJ/mol for Cu@Co NPs [20],
54.5 kJ/mol for RuePd@PVP NPs [21], 56.3 kJ/mol for PteRu@PVP
NPs [22], and 51.7 kJ/mol for PdePt@PVP NPs [24]; but still higher
than 37.2 kJ/mol for NieRu NPs [5], 36.6 kJ/mol for Ru@Ni NPs [8],
38 kJ/mol for CuNi NPs [9], 45 kJ/mol for NiPd NPs [18], and 46.1 kJ/
mol for PdeRh@PVP NPs [23].
Fig. 5 shows the plots of the volume of generated hydrogen gas
versus time in the catalytic hydrolysis of 100 mM AB solutions in
the presence of RueRh@PVP nanoparticles (0.3 mM) at various
temperatures (283, 288, 293, 298, and 303 K). It is worth to note
that using RueRh@PVP nanoparticles (0.3 mM) leads to complete
hydrogen release (3.0 mol H2/mol AB) for the hydrolysis of AB
within 210 s, corresponding to an average TOF value of 386 mol H2
(mol cat)ꢀ1 minꢀ1 at 298 K. This average TOF value is among the
best TOF values provided some bimetallic catalysts in the hydrolysis
of AB (Table 1). Higher catalytic activity of RueRh@PVP nano-
particles stems from the synergistic effects of ruthenium and
rhodium and the reduced particle size of the catalyst.
The rate constants of hydrogen generation from the hydrolysis
of AB were calculated from the linear portions of the plots given in
Fig. 5 at five different temperatures and used for the calculation of
the activation energy (Ea ¼ 47.4 2.1 kJ/mol for the hydrolysis of
AB) from the Arrhenius plot (Inset in Fig. 5) for hydrolysis reaction.
This value of activation energy for the hydrolysis of AB is lower than
3.3. The durability of RueRh@PVP nanoparticles in the hydrolysis of
AB
The durability of RueRh@PVP nanoparticles in the hydrolysis of
AB was investigated by successive additions of AB after the first
cycle of the hydrolysis reaction. The RueRh@PVP nanoparticles
catalyst retains 68% of its initial catalytic activity in the hydrolysis of
AB, even at the fifth run (Fig. 6). The decrease in the catalytic ac-
tivity of RueRh@PVP nanoparticles in the hydrolysis of AB is due to
the passivation of the surface of nanoparticles by increasing
amount of metaborate, which decreases accessibility of active sites
[30] and the aggregation of nanoparticles as shown in the TEM
image of the catalyst taken after fifth run of the hydrolysis reaction
(Fig. 2d, scale bar represents 5 nm).
4. Conclusions
In summary, the study of the preparation, characterization and
employment of RueRh@PVP nanoparticles as catalyst for the hy-
drolysis of AB has led to the following conclusions and insights:
✓ RueRh@PVP nanoparticles can be easily prepared from the co-
reduction of corresponding ruthenium and rhodium salts by
an alcohol reduction method.
✓ RueRh@PVP nanoparticles are highly efficient catalyst for
hydrogen generation from the hydrolysis of AB providing high
catalytic activity.
✓ They provide average TOF value of 386 mol H2 (mol cat)ꢀ1 minꢀ1
and maximum hydrogen generation rate of 10,680 L H2 minꢀ1
(mol cat)ꢀ1 for the hydrolysis of AB.
✓ Activation energy for the catalytic hydrolysis of AB in the pres-
ence of RueRh@PVP nanoparticles was calculated as
47.4 2.1 kJ/mol.
Fig. 5. Plots of the volume of generated hydrogen gas versus time in the catalytic
hydrolysis of 100 mM AB solutions in the presence of RueRh@PVP nanoparticles
(0.3 mM) at various temperatures (283, 288, 293, 298, and 303 K). Inset shows the
Arrhenius plot for the hydrolysis of AB (100 mM) catalyzed by 0.3 mM RueRh@PVP
nanoparticles.
Table 1
Activities in terms of TOF values of some bimetallic catalyst systems tested in
hydrogen generation from the hydrolysis of AB.
Catalyst
TOF (mol H2.mol catalystꢀ1.minꢀ1
)
Reference
Ni@Ru NPs
RuCu NPs
Ru@Ni NPs
CueNi NPs
114.0
135.0
340.0
10.7
[4]
[7]
[8]
[9]
Ru@Co NPs
CoNi NPs
CuCo NPs
Ag/Pd NPs
AuCo NPs
PteCo NPs
NiPd NPs
CueNi NPs
Cu@Co NPs
RuePd@PVP NPs
RuePt@PVP NPs
PdeRh@PVP NPs
PdePt@PVP NPs
RueRh@PVP NPs
344.0
19.5
19.6
6.3
23.5
377.8
28.7
60.0
[10]
[11]
[12]
[14]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[This study]
8.4
308.0
308.0
1333.0
125.0
386.0
Fig. 6. Retained % initial catalytic activity of 0.3 mM RueRh@PVP nanoparticles in the
successive catalytic runs for the hydrolysis of 0.100 M AB at 298 K.

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M. Rakap / Journal of Alloys and Compounds 649 (2015) 1025e1030
[10] N. Cao, J. Su, W. Luo, G. Cheng, Catal. Commun. 43 (2014) 47e51.
[11] Y. Yang, F. Zhang, H. Wang, Q. Yao, X. Chen, Z.H. Lu, J. Nanomater. http://
dx.doi.org/10.1155/2014/294350.
[12] J. Li, Q.L. Zhu, Q. Xu, Catal. Sci. Technol. 5 (2015) 525e530.
[13] W. Jiao, X. Hu, H. Ren, P. Xu, R. Yu, J. Chen, X. Xing, J. Mater. Chem. A 2 (2014)
18171e18176.
✓ RueRh@PVP nanoparticles can be regarded as promising cata-
lysts having high activity for practical applications to supply
hydrogen from the hydrolysis of AB for proton exchange mem-
brane fuel cells.
[14] Y. Tong, X. Lu, W. Sun, G. Nie, L. Yang, C. Wang, J. Power Sources 261 (2014)
221e226.
Acknowledgments
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Interfaces 6 (2014) 12429e12435.
This study is supported by Research Fund of Yuzuncu Yil Uni-
versity (Project No: BAP-2013-FEN-B-014).
[16] J. Li, Q.L. Zhu, Q. Xu, Chem. Commun. 50 (2014) 5899e5901.
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[20] Y. Du, N. Cao, L. Yang, W. Luo, G. Cheng, New J. Chem. 37 (2013) 3035e3042.
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