Graphene supported Ru@Co core-shell nanoparticles as efficient catalysts for hydrogen generation from hydrolysis of ammonia borane and methylamine borane

Article abstract of DOI:10.1016/j.catcom.2013.09.003

Well-dispersed graphene supported Ru@Co core-shell nanoparticles were synthesized by one-step in situ co-reduction of aqueous solution of ruthenium(III) chloride hydrate, cobalt(II) chloride hexahydrate and graphite oxide (GO) with ammonia borane under ambient condition. The as-synthesized nanoparticles exert excellent catalytic activities, with the turnover frequency (TOF) value of 344 mol H₂ min^{- 1} (mol Ru)^{- 1} for catalytic hydrolysis of ammonia borane, which is the second highest value ever reported. The as-synthesized catalysts exert superior catalytic activities than the monometallic (Ru/graphene), alloy (RuCo/graphene), and graphene-free Ru@Co counterparts towards the hydrolytic dehydrogenation of AB. Moreover, the catalytic hydrolysis of MeAB at room temperature was also studied. These Ru@Co NPs are a promising catalyst for amine-borane hydrolysis and for developing a highly efficient hydrogen storage system for fuel cell applications.

Full text of DOI:10.1016/j.catcom.2013.09.003

Graphene supported Ru@Co core-shell nanoparticles as efficient catalysts for
hydrogen generation from hydrolysis of ammonia borane and methylamine
borane
Nan Cao, Jun Su, Wei Luo, Gongzhen Cheng
PII:
DOI:
Reference:
S1566-7367(13)00339-7
doi: 10.1016/j.catcom.2013.09.003
CATCOM 3634
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
19 July 2013
29 August 2013
2 September 2013
Please cite this article as: Nan Cao, Jun Su, Wei Luo, Gongzhen Cheng, Graphene
supported Ru@Co core-shell nanoparticles as efficient catalysts for hydrogen generation
from hydrolysis of ammonia borane and methylamine borane, Catalysis Communications
(2013), doi: 10.1016/j.catcom.2013.09.003
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ACCEPTED MANUSCRIPT
Graphene supported Ru@Co core-shell nanoparticles as efficient catalysts for
hydrogen generation from hydrolysis of ammonia borane and methylamine
borane
a
b
a
a
Nan Cao, Jun Su, Wei Luo* and Gongzhen Cheng
a
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei
4
30072, P. R. China. E-mail address: wluo@whu.edu.cn
b
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science
and Technology, Wuhan, Hubei, 430074, P. R. China
Abstract
Well-dispersed graphene supported Ru@Co core-shell nanoparticles were synthesized
by one-step in situ co-reduction of aqueous solution of ruthenium(III) chloride
hydrate, cobalt(II) chloride hexahydrate and Graphite Oxide (GO) with ammonia
borane under ambient condition. The as-synthesized nanoparticles exert excellent
-1
catalytic activities, with the turnover frequency (TOF) value of 344 mol H min (mol
2
-1
Ru) for catalytic hydrolysis of ammonia borane, which is the second highest value
ever reported. The as-synthesized catalysts exert superior catalytic activities than the
monometallic (Ru/graphene), alloy (RuCo/graphene), and graphene-free Ru@Co
counterparts towards the hydrolytic dehydrogenation of AB. Moreover, the catalytic
hydrolysis of MeAB at room temperature was also studied. These Ru@Co NPs are a
promising catalyst for amine-borane hydrolysis and for developing a highly efficient
hydrogen storage system for fuel cell applications.
1

ACCEPTED MANUSCRIPT
Keywords: core-shell, ruthenium, cobalt, graphene, hydrogen storage
1
. Introduction
Safe and efficient storage of hydrogen is essential for the development of a
hydrogen-based energy infrastructure [1]. Recently, numerous hydrogen storage
approaches are currently under investigation, including metal hydrides [2], sorbent
materials [3], and chemical hydride systems [4]. Among the chemical hydrides,
ammonia borane (NH -BH , AB) has recently received a great interest because of its
3
3
1
9.6 wt% hydrogen content, highly stability, and environmentally benign [5].
Hydrolytic dehydrogenation of AB is considered to be the most convenient approach
for portable hydrogen storage application [6]. However, the methyl-substituted AB,
methylamine borane (CH NH -BH , MeAB) with the 11.1 wt% hydrogen content, has
3
2
3
not been widely studied [7]. Furthermore, to the best of our knowledge, the hydrolysis
of MeAB, which could also release 3 mol H per mol MeAB at room temperature has
2
been rarely reported. Therefore, searching for suitable catalysts that meet efficient,
economical, and stable requirements toward hydrogen generation from the
amine-borane systems under moderate conditions, is crucial for their practical
applications.
On the other hand, bimetallic transition metal core-shell nanoparticles (NPs) have
attracted considerable interest owing to their unique optical properties [8], electronic
properties [9], and exciting potential for application in biological [10], chemical
sensing [11], optoelectronics [12], magnetics [13], and catalysis [14]. Such materials
2

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are often found to possess enhanced catalytic properties in contrast to the
monometallic counterparts and alloys, due to the interplay of electronic and lattice
effects of the neighboring metals [15]. Ru nanoparticles, including monometallic
Ru-based NPs [16] and bimetallic RuCo [17], RuNi [18], and Ni@Ru [19] NPs, have
showed extremely high activities for catalytic dehydrogenation of AB. Recently, the
synthesis of Ru@Ni core-shell NPs has been reported by a spray-pyrolysis method
[
20], however, as far as we know, there are no reports about synthesis and
characterization of Ru@Co core-shell NPs. Therefore, developing a facile one-step
route to construct the Ru-based core-shell NPs with high catalytic activities are highly
desirable.
Herein, we first report the in situ synthesis of magnetically recyclable graphene
supported Ru@Co core-shell NPs using AB as reductant in a one-step co-reduction
route at room temperature under ambient atmosphere. Graphene was chosen as a
support because it has been reported to be a suitable substrate to grow active materials
and maintain the stability of the metal NPs during the catalytic process [21], due to its
outstanding charge carrier mobility [22], thermal and chemical stability [23], and high
specific surface area [24], etc.. The as-synthesized catalysts exert superior catalytic
activities than the monometallic (Ru/graphene), alloy (RuCo/graphene), and
graphene-free Ru@Co counterparts towards the hydrolytic dehydrogenation of AB.
Moreover, the catalytic hydrolysis of MeAB at room temperature was also studied.
2
. Experimental
The detail experimental process could be found in the supporting information.
3

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3
3
. Results and Discussion
.1 Synthesis and characterization
The graphene supported core-shell Ru@Co/graphene NPs were prepared by adding
AB into the precursor solution containing RuCl , CoCl , and graphene oxide (GO) at
3
2
3
+
2+
room temperature. The Ru and Co were reduced in sequence to produce core-shell
structured NPs during the reduction process. Considering the reduction potentials of
3
+
2+
0
3+
0
2+
Ru and Co ( E (Ru /Ru) = +0.40 eV vs. SHE; E (Co /Co) = -0.28 eV vs. SHE),
3
+
Ru with higher reduction potential was first reduced by AB, and formed the in situ
seed to induce the successive growth of the Co as the shell.
The microstructure of Ru@Co/graphene NPs were characterized by transmission
electron microscopy (TEM). As shown in Fig. 1a, the NPs were well dispersed on
graphene, which help to prevent the agglomeration. In Fig. 1b, there are three kinds of
NPs, the big particles are around ~15 nm, some of them are monometallic Co NPs,
while the others are bimetallic Ru@Co NPs, with the core-shell structures; the small
ones with the particle sizes around ~2 nm are monometallic Ru NPs. The high
resolution TEM (HRTEM) image indicates the core-shell NPs are mostly amorphous,
and a distinct contrast of core and shell can be observed clearly in Fig. 1c. The dark
core is Ru, and the gray shell is Co, indicating that Ru is initially reducing by AB, and
subsequently acting as the seed helping for the reducing Co to form the shell. The
lattice fringes of the small NPs can be clearly observed, with the lattice separation is
0
.234 nm, which is consistent with the (100) plane of Ru (Fig. 1d). Therefore, it can
be conclude that the as-synthesized NPs are the composite of the Ru, Co, and Ru@Co
4

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core-shell NPs [25]. This may be caused by the discontinuous growth of Co shell on
parts of the Ru seeds. Due to the small amount of Ru, the X-ray diffraction (XRD)
pattern (Fig. S1) of the as-synthesized NPs has no obvious diffraction peak, indicating
the sample is almost in an amorphous state, which is in good agreement with the TEM
results. Furthermore, the most intense peak at around 8.3° corresponds to the GO
disappeared, and new broad peak around 23.8° appeared in the as-prepared NPs
indicating that the GO is successfully reduced to graphene. In the Raman
spectroscopy (Fig. S2), the GO and graphene supported Ru@Co exhibit two peaks
-1
centered at 1354 and 1608 cm , corresponding to the D and G bands of the carbon
products, respectively. The intensity ratio of the D to G band (I /I ) is generally
D
G
accepted to reflect the degree of graphitization of carbonaceous materials and defect
density. After loading of the as-synthesized NPs, the I /I of GO is increased from 1.0
D
G
to 1.8. The relative changes in the D to G peak intensity ratio confirm the reduction of
GO during the in situ fabrication. It can be seen clearly from the FTIR spectra (Fig.
-1
-1
S3) that the disappearance of C=O peak at 1736 cm , C-OH peak at 1224 cm and
-1
the C-O peak at 1052 cm of GO after the formation of the as-synthesized NPs,
further indicating the GO was reduced to graphene during the process.
3
.2 Catalytic activities for hydrolysis of AB
3+
2+
Without Ru , the Co cannot be reduced by AB, resulting in no activity toward
catalytic hydrolysis of AB as shown in Fig. 2. When Ru NPs were supported on
graphene, keeping the molar ratio of Ru/AB as 0.004, AB cannot be catalytic
decomposed completely, and only 89% of H is released for about 1 h. Unexpectedly,
2
5

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when Co was introduced into the NPs to form the core-shell NPs, and still keeping the
molar ratio of Ru/AB as 0.004, the decomposed of AB was completed, and the
reaction time was decreased. When the Co/Ru molar ratio is 7.5, the release of H can
2
be completed within 2 min, further increased the Co/Ru molar ratio to 10 results in the
prolonging of the reaction time, indicating the positive effect of the Ru@Co core-shell
nanostructures to the catalytic hydrolysis of AB, and the best Co/Ru molar ratio in the
Ru@Co core-shell NPs is 7.5. The activity in terms of turnover frequency (TOF) is
-1
-1
3
44 (mol H min (mol Ru) ) for this as-synthesized NPs, moreover, to the best of
2
our knowledge, this is the second highest value ever reported for the catalytic
hydrolysis of AB among the Ru-based catalyst, and higher than most of other reported
noble metal-based catalysts, if the TOF is normalized in terms of mol noble metal
(Table 1). Interestingly, the TOF value of the as-synthesized NPs is much higher than
the Ni@Ru core-shell NPs, which have a more reactive Ru as the shell [19]. The
results may be attributed to the positive synergistic effect between Ru and Co in the
core-shell nanostructure, and more active sites deriving from the higher amorphous
Co contents [21].
For comparison, RuCo/graphene and graphene-free Ru@Co NPs are also prepared
and their catalytic activities to hydrolysis of AB are studied. As shown in Fig. 3, their
catalytic activities are both inferior to that of Ru@Co/graphene NPs, which confirms
that the positive synergistic effect of Ru@Co core-shell nanostructure. Furthermore,
the catalytic activities of RuCo/graphene NPs are better than that of the Ru@Co NPs,
highlighting the cooperative effect between graphene and NPs for AB
6

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dehydrogenation.
.3 Recycle ability
The recyclability of the catalyst is crucial in the practical application. The
3
recyclability of as-synthesized composite of Ru/graphene and Ru@Co/graphene NPs
up to fifth run for hydrolysis of AB and MeAB are shown in Fig. 4 and 5 respectively.
The as-prepared catalysts retain 51% of their initial catalytic activities in the
hydrolysis of AB in the fifth run. In Fig. 5, with our as-synthesized NPs, MeAB can
release 3 equiv. H in less than 4 min at room temperature, with the TOF measured to
2
-1
-1
be 226 (mol H min (mol Ru) ), and retain 51% of their initial catalytic activities
2
after fifth run. Furthermore, the in situ synthesized NPs are magnetic and thus can be
separated from the reaction solution by an external magnet (Fig. 5 inset), which
makes the practical recycling application of the NPs more convenient. Additionally,
when the recycled cycles increased to ten, there are only 32% and 25% of their initial
catalytic activities were left for the catalytic hydrolysis of AB and MeAB,
respectively (Fig. S4, S5). Further study about increase the stability and recyclability
of the as-synthesized catalysts should be done toward their practical application. Fig.
1
d and Fig. 1e show the representative TEM images of the as-synthesized NPs after
the fifth run durability test. As clearly seen from the TEM images, there is no
noticeable change in the morphology, and no obvious aggregation of the Ru NPs on
graphene, but the Ru@Co core-shell NPs are readily agglomerate on graphene after
five cycles. Therefore, the observed decrease in the catalytic activity may be
attributed to the agglomeration of Ru@Co core-shell NPs, highlighting the positive
7

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effect of Ru@Co core-shell NPs toward to catalytic hydrolysis of AB and MeAB.
Further study about synthesis of the pure Ru@Co core-shell NPs is in process in our
lab, which are supposed to exert higher catalytic activities for the hydrolytic
dehydrogenation of AB and MeAB.
To better understand the hydrolysis of MeAB in the presence of as-synthesized NPs,
and confirm the completely decomposed of MeAB after the catalytic reaction, we
11
11
performed the B NMR studies (Fig. S4). The B peak at -19.8 ppm which assigned
to MeAB is disappeared after the catalytic reaction, and a new peak around 11.4 ppm
is observed indicating three moles of hydrogen has been generated from the one mole
of MeAB via the catalytic hydrolysis according to Eq. (1). Furthermore, there is no
11
change in the B NMR of the MeAB after stay 7 days under ambient condition,
indicating the catalytic activities of the as-prepared NPs toward catalytic hydrolysis of
MeAB at ambient condition.
catalyst
MeNH -BH + 2 H O
(
MeNH )BO + 3 H (1)
3 2
2
2
3
2
Conclusion
In summary, for the first time, the Ru@Co core-shell NPs were synthesized by a
one-step in situ method using graphene as the supported material. The as-synthesized
NPs exhibit superior catalytic activities than the monometallic Ru and Co, alloy RuCo,
and graphene-free Ru@Co counterparts. The turnover frequency (TOF) value of the
-1
-1
as-synthesized NPs is 344 mol H min (mol Ru) for catalytic hydrolysis of
2
ammonia borane, which is the second highest value ever reported. In addition, the
as-synthesized NPs show good durable stability and magnetically recyclability for the
8

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hydrolytic dehydrogenation of AB and MeAB. Moreover, this simple synthetic
method can be extend to other Ru-based bimetallic or trimetallic core-shell systems
for more applications.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of
China (21201134), the Ph. D Programs Foundation of Ministry of Education of China
(20120141120034), and Large-scale Instrument and Equipment Sharing Foundation
of Wuhan University.
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Figure Captions
Table 1. TOF values for hydrolysis of AB catalyzed by different catalysts.
Fig. 1 (a)-(d) TEM images of the composite of Ru/graphene and Ru@Co/graphene
NPs with different magnifications; (e)-(f) TEM images of the as-synthesized NPs after
the fifth cycle.
Fig. 2 Time plots of catalytic dehydrogenation of AB over the as-synthesized catalysts
at ambient condition. Ru/AB = 0.004 (molar ratio).
Fig. 3 Time plots of catalytic dehydrogenation of AB over RuCo/graphene, composite
of Ru/graphene and Ru@Co/graphene, and composite of Ru and Ru@Co catalysts at
ambient condition. Ru/AB = 0.004, Co/Ru = 7.5.
Fig. 4 Time plots of catalytic dehydrogenation of AB over the as-synthesized catalysts
o
from 1st to 5th cycle at 25 ± 0.2 C. Ru/AB = 0.004, Co/Ru = 7.5.
Fig. 5 Time plots of catalytic dehydrogenation of MeAB over the as-synthesized
o
catalysts from 1st to 5th cycle at 25 ± 0.2 C. Ru/MeAB = 0.004, Co/Ru = 7.5.
12

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Table 1 TOF values for hydrolysis of AB catalyzed by different catalysts.
-1
-1
Catalyst
TOF(mol H mol M min )
Ref.
2
M=Pt, Ru, Pd, Au
429.5
Ru/C
16(a)
Pd@Co/graphene
Ru@Co/graphene
Ru(0)@MWCNT
Ni0.33@Pt0.67/C
408.9
344
329
280.3
21
This study
16(b)
26
Pt@MIL-101
258.6
27
Co0.32@Pt0.68/C
Ni0.74Ru0.26 alloy NPs
PSSA-co-MA stabilized Ru
nanoclusters
248.5
194.8
187.6
28
18
29
Ni@Ru
Ru@Al O after acetic acid
114
83.3
16(b)
30
2
3
treatment
Ru@Al O
RuCo (1:1)/γ-Al O
PSSA-co-MA stabilized Pd
nanoclusters
39.6
32.9
19.9
30
17
29
2
3
2
3
RuCu (1:1)/γ-Al O
16.4
17
2
3
13

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Fig. 1 (a)-(d) TEM images of the composite of Ru and Ru@Co/graphene NPs with
different magnifications; (e)-(f) TEM images of the as-synthesized NPs after the fifth
cycle.
Co/Ru=2.5
Co/Ru=5
Co/Ru=7.5
Co/Ru=10
Ru(0.004mmol)
Co(0.03mmol)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
2
4
6
8
10
time(min)
Fig. 2 Time plots of catalytic dehydrogenation of AB over the as-synthesized catalysts
at ambient condition. Ru/AB = 0.004 (molar ratio).
14

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AB(with GO)
SB(with GO)
AB(without GO)
3
2
2
1
1
0
0
.0
.5
.0
.5
.0
.5
.0
0
1
2
3
4
5
6
7
8
time(min)
Fig. 3 Time plots of catalytic dehydrogenation of AB over RuCo/graphene, composite
of Ru/graphene and Ru@Co/graphene, and composite of Ru and Ru@Co catalysts at
ambient condition. Ru/AB = 0.004, Co/Ru = 7.5.
Fig. 4 Time plots of catalytic dehydrogenation of AB over the as-synthesized catalysts
o
from 1st to 5th cycle at 25 ± 0.2 C. Ru/AB = 0.004, Co/Ru = 7.5.
Fig. 5 Time plots of catalytic dehydrogenation of MeAB over the as-synthesized
o
catalysts from 1st to 5th cycle at 25 ± 0.2 C. Ru/MeAB = 0.004, Co/Ru = 7.5.
15

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Graphical abstract
Well dispersed Ru@Co/graphene nanoparticles were synthesized via a one-step in situ
co-reduction route. The as-synthesized NPs exerted satisfied catalytic activity and
recycle stability towards the hydrolysis of ammonia borane and methylamine borane.
16

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Highlights:
First synthesize Ru@Co core-shell NPs supported on graphene
The catalysts exhibit excellent catalytic activity toward hydrolysis of AB and MeAB
Second highest TOF value for the catalytic hydrolysis of AB
17