Size-controlled synthesis of tetrametallic Ag@CoNiFe core-shell nanoparticles supported on graphene: A highly efficient catalyst for the hydrolytic dehydrogenation of amine boranes

Article abstract of DOI:10.1002/cctc.201400042

Tetrametallic core-shell Ag@CoNiFe nanoparticles (NPs) supported on graphene have been synthesized by a facile one-step in situ procedure for the first time. We tuned the reducing ability of the reductants to control the structure and particle size of the as-synthesized NPs. The use of weaker reducing agents, such as ammonia borane (AB) and methylamine borane (MeAB), result in the core-shell structure, whereas the stronger reducing agent NaBH₄ results in alloy NPs. The as-synthesized core-shell NPs reduced by MeAB possess a smaller particle size and exhibit a superior catalytic activity compared to the core-shell NPs reduced by AB and alloy NPs reduced by NaBH₄ for the hydrolytic dehydrogenation of AB. Moreover, the turnover frequency of the as-synthesized tetrametallic core-shell NPs is 118.5 mol H 2 min^-1 mol_Ag^-1, which is higher than that of Ag-based trimetallic, bimetallic core-shell, and alloy counterparts and even higher than that of most reported noble-metal-based NPs. Kinetic studies indicate that the catalytic hydrolysis of AB and MeAB by the Ag@CoNiFe/graphene NPs is a first-order reaction. Furthermore, the as-prepared NPs exert good catalytic activities and recycle stabilities towards the hydrolysis of AB. Additionally, the as-prepared NPs display magnetic recyclability, which makes the practical recycling of the NPs more convenient. Core-shell casts a spell: Well-dispersed tetrametallic core-shell Ag@CoNiFe/graphene nanoparticles exert a satisfactory catalytic activity and recyclability towards the hydrolysis of ammonia borane and methylamine borane.

Full text of DOI:10.1002/cctc.201400042

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DOI: 10.1002/cctc.201400042
Size-Controlled Synthesis of Tetrametallic Ag@CoNiFe
Core–Shell Nanoparticles Supported on Graphene:
A Highly Efficient Catalyst for the Hydrolytic
Dehydrogenation of Amine Boranes
Lan Yang,[a] Jun Su,[c] Wei Luo,*[a, b] and Gongzhen Cheng[a]
Tetrametallic core–shell Ag@CoNiFe nanoparticles (NPs) sup-
ported on graphene have been synthesized by a facile one-
step in situ procedure for the first time. We tuned the reducing
ability of the reductants to control the structure and particle
size of the as-synthesized NPs. The use of weaker reducing
agents, such as ammonia borane (AB) and methylamine
borane (MeAB), result in the core–shell structure, whereas the
stronger reducing agent NaBH4 results in alloy NPs. The as-syn-
thesized core–shell NPs reduced by MeAB possess a smaller
particle size and exhibit a superior catalytic activity compared
to the core–shell NPs reduced by AB and alloy NPs reduced by
NaBH4 for the hydrolytic dehydrogenation of AB. Moreover,
the turnover frequency of the as-synthesized tetrametallic
core–shell NPs is 118.5 molH minÀ1 molAgÀ1, which is higher
2
than that of Ag-based trimetallic, bimetallic core–shell, and
alloy counterparts and even higher than that of most reported
noble-metal-based NPs. Kinetic studies indicate that the cata-
lytic hydrolysis of AB and MeAB by the Ag@CoNiFe/graphene
NPs is a first-order reaction. Furthermore, the as-prepared NPs
exert good catalytic activities and recycle stabilities towards
the hydrolysis of AB. Additionally, the as-prepared NPs display
magnetic recyclability, which makes the practical recycling of
the NPs more convenient.
Introduction
Heterometallic nanoparticles (NPs) have attracted increasing at-
tention because of their fascinating applications in a variety of
fields, such as chemical sensing,[1] drug delivery,[2] plasmonics,[3]
optoelectronics,[4] magnetic memory,[5] and catalysis.[6] Among
them, core–shell architectures have attracted more attention in
the application of nanocatalysts because of their new and effi-
cient catalytic activity compared with their monometallic coun-
terparts and alloys.[7] Tremendous efforts have been devoted
to the size- and shape-controlled synthesis of bimetallic core–
shell NPs, however, to date, the controlled synthesis of multi-
ple metal NPs (nꢀ3) has been relatively unexplored. For in-
stance, Wang and Yamanchi,[8] Xu et al.,[9] Sun et al.,[10] and Yan
et al.[11] have developed different strategies to obtain trimetallic
core–shell NPs. To the best of our knowledge, reports on tetra-
metallic core–shell NPs are rare.[12] Therefore, to investigate the
possible applications of multimetallic NPs in comparison to bi-
metallic and monometallic NPs, the development of an effi-
cient method to produce multimetallic NPs with a desirable
structure is of great importance.
The safe and efficient storage of hydrogen is still a major
hurdle for a prospective hydrogen economy.[13] Over the past
few decades, many methods have been proposed for the stor-
age of hydrogen, among them, amine borane compounds
have attracted much attention because of their high gravimet-
ric hydrogen densities and favorable kinetics of hydrogen re-
lease.[14] As the parent amine borane, ammonia borane (AB)
has been considered to be the most promising candidate for
on-board hydrogen storage applications because of its
19.6 wt% hydrogen content, high stability, and environmental
benignity.[15] The hydrogen stored in AB can be released
through different ways,[16] but the hydrolysis of AB by transi-
tion-metal-nanocatalysts under ambient conditions seems to
be the most promising route for on-board applications.[17] So
far, not only monometallic NPs[18] but also bimetallic[19] and tri-
metallic NPs[9,11] have been tested for the catalytic hydrolysis of
AB. It is well known that the catalytic process takes place on
the metal surfaces, on which the smaller metal NPs have
higher surface areas and exhibit higher catalytic activity.[20]
As classical Lewis acid–base adducts, amine boranes have
weak reducibility, which offers much better control of the re-
duction rate and selective reduction of metal ions over
traditional strong reducing agents such as borohydrides. The
reducing strength decreases if the alkyl substitution on the ni-
trogen atom is increased: H3NBH3 >MeNH2BH3 >Me2NHBH3 >
[a] L. Yang, Prof. Dr. W. Luo, Prof. Dr. G. Cheng
College of Chemistry and Molecular Sciences
Wuhan University
Wuhan, Hubei 430072 (P. R. China)
Tel : (+86)2768752366
E-mail: wluo@whu.edu.cn
[b] Prof. Dr. W. Luo
Suzhou Institute of Wuhan University
Suzhou, Jiangsu, 215123 (P. R. China)
[c] Dr. J. Su
Wuhan National Laboratory for Optoelectronics
Huazhong University of Science and Technology
Wuhan, Hubei, 430074 (P. R. China)
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Me3NBH3.[21] Recently, we have succeeded in the production of
graphene-supported bimetallic Ag@Co, Ag@Ni, and Ag@Fe
core–shell NPs[22] and trimetallic Ag@CoNi, Ag@CoFe, and
Ag@NiFe core–shell NPs by using amine boranes as the reduc-
tant.[23,24] The trimetallic core–shell NPs exhibit superior catalyt-
ic activity towards the hydrolysis of AB than the bimetallic
core–shell NPs and monometallic counterparts.
Herein, we report a facile method for the size-controlled syn-
thesis of graphene-supported tetrametallic Ag@FeCoNi core–
shell NPs for the first time. Graphene was chosen as the sup-
port because of its superior electrical conductivity, high me-
chanical strength, chemical stability, and high surface-to-
volume ratio.[25,26] It has been demonstrated that the enhanced
catalytic activity of graphene-supported metal NPs is caused
mainly by charge transfer across the graphene–metal interface
caused by the graphene–metal spacing and Fermi level differ-
ence.[27] Interestingly, the tetrametallic core–shell NPs exhibit
higher catalytic activity than the trimetallic and bimetallic Ag-
based core–shell NPs towards the hydrolysis of AB. The catalyt-
ic hydrolysis of methylamine boran (MeAB) was also studied.
Results and Discussion
Synthesis and characterization
In a typical synthesis of graphene-supported Ag@CoNiFe core–
shell NPs, an aqueous solution of AB, MeAB, or NaBH4 was in-
troduced into a round-bottomed flask that contained an aque-
ous solution of AgNO3, CoCl2, NiCl2, FeCl2, and graphene oxide
(GO). The Ag+ with a high reduction potential (E0(Ag+/Ag)=
+0.80 eV vs. the standard hydrogen electrode (SHE)) was first
reduced by AB or MeAB to form the Ag NPs to serve as the
in situ seeds to induce the successive growth of the rest of the
metal NPs as the shell, which may be generated by AgÀH spe-
cies or H2 released from AB/MeAB catalyzed by the Ag
seeds[9,28,29] to form the Ag@CoNiFe core–shell NPs. Without
Ag cores, Co2+, Ni2+, and Fe2+ cannot be reduced by AB or
MeAB directly because of their low reduction potentials
(E0(Co2+/Co)=À0.28 eV vs. SHE; E0(Ni2+/Ni)=À0.25 eV vs. SHE;
E0(Fe2+/Fe)=À0.44 eV vs. SHE) and the weak reduction capabil-
ity of AB/MeAB. If a stronger reducing agent such as NaBH4
was used, the metal precursors were reduced simultaneously
to form AgCoNiFe alloy NPs.
Figure 1. a) and b) TEM images of Ag@CoNiFe/graphene NPs; c) and d) TEM
images of NPs reduced by AB and NaBH4; e) and f) TEM image of Ag@Co-
NiFe NPs without support; g) TEM image of Ag@CoNiFe/graphene NPs after
five cycles; h) EDS spectrum of Ag@CoNiFe/graphene NPs; a)–c) inset: parti-
cle size of Ag@CoNiFe/graphene NPs reduced by MeAB, AB, and NaBH4, re-
spectively.
The microstructures of Ag0.1@[(Co0.5Ni0.5)0.9Fe0.1]0.9/graphene
NPs reduced by AB and MeAB were characterized by TEM. A
distinct contrast between the core and shell can be observed
clearly in Figure 1a and c; the as-synthesized NPs were well
dispersed on graphene, which helps to prevent their agglom-
eration. The high-resolution transmission electron microscopy
(HRTEM) image is shown in Figure 1b; the d spacing of the
crystallized core is ~0.235 nm, which is consistent with the
spacing of the Ag (111) plane. This result indicates that Ag is
reduced initially by MeAB or AB, and subsequently acts as the
seed to reduce Co, Ni, and Fe to form the shell. The energy-dis-
persive X-ray (EDS) spectrum shown in Figure 1h confirms the
presence of Ag, Co, Ni, and Fe, and the molar ratio of the four
metals is 10:44:39:7, which is close to the target value. If
a stronger reducing agent, NaBH4, was used, AgCoNiFe/gra-
phene alloy NPs were formed as shown in Figure 1d. The
mean diameter of the as-synthesized NPs in Ag@CoNiFe/gra-
phene reduced by MeAB was around 12 nm, in Ag@CoNiFe/
graphene reduced by AB it was around 15 nm, and in AgCo-
NiFe/graphene reduced by NaBH4 it was around 20 nm. This
result indicates that the use of weaker reducing agents could
result in much slower reduction kinetics and more size control
over the nucleation of the NPs to turn the nanostructures from
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alloy to core–shell, which could affect their catalytic activity
(vide infra). A TEM image of as-synthesized core–shell NPs re-
duced by MeAB without graphene is shown in Figure 1e and f.
It is clear that without the support of graphene, the NPs exhib-
it severe agglomeration.
The formation process of the NPs has been further examined
and monitored by UV/Vis spectroscopy by withdrawing sam-
ples at different stages of the reaction, and a mechanism is
proposed as illustrated in Figure 2. The UV/Vis spectrum of the
Figure 3. XRD patterns of a) GO, b)–d) graphene-supported AgCoNiFe NPs
reduced by MeAB, AB, and NaBH4; e) NPs without support reduced by
MeAB, f) graphene-supported NPs reduced by NaBH4 after they had been
annealed at 5008C for 4 h under a N2 atmosphere.
Figure 2. Schematic view of the formation of the Ag@CoNiFe NPs monitored
by UV/Vis spectroscopy a) before adding the reductant MeAB, b) during the
reaction, and c) after the reaction finished.
solution before the reductant was added is shown in Fig-
ure 2(a), and there is no peak in the spectrum. A distinct peak
around l=400 nm is seen in the spectrum shown in Fig-
ure 2b,[30] which arises from the formation of Ag seeds on the
addition of MeAB, and this characteristic peak almost disap-
pears completely by the end of reaction (c) for Ag@CoNiFe
NPs covered fully with Co, Ni, and Fe, which is in accord with
the results obtained from the TEM images. The powder XRD
patterns of the as-prepared graphene-supported NPs reduced
by MeAB and AB and Ag@CoNiFe without support by gra-
phene reduced by MeAB are shown in Figure 3b, c, and e, re-
spectively. There are two peaks at 2q=38.1 and 44.38, which is
in good agreement with the values for Ag (111) and Ag (200).
However, no diffraction peaks of Co, Ni, and Fe are present,
which may be because of the amorphous phase of Co, Ni, and
Fe as indicated in the TEM images. The XRD pattern of the as-
prepared graphene-supported NPs reduced by NaBH4 is shown
in Figure 3d, and there are no obvious peaks. The XRD pattern
of NPs reduced by NaBH4 after they were annealed at 5008C
for 4 h under a N2 atmosphere is shown in Figure 3 f, and the
intensity of the peaks is increased distinctly. The peak at 2q=
38.18 is attributed to Ag (111), the peak that results from Ag
(200) is shifted slightly to higher angles, and the peak at 2q=
51.78 is a little higher that of than Co (200). These phenomena
indicate that the catalysts reduced by NaBH4 are alloys, which
is in accord with the strong reducibility of NaBH4. Furthermore,
the most intense peak at around 2q=9.68 that correspond to
GO disappears, and new peaks at around 2q=24.48 are ob-
served in the as-prepared graphene-supported NPs, which indi-
cates that the GO is reduced successfully to graphene. In the
Raman spectra (Figure 4), the GO and graphene-supported NPs
Figure 4. Raman spectra of GO and Ag@CoNiFe/graphene NPs.
exhibit two peaks centered at n˜ =1358 and 1596 cmÀ1, which
correspond to the D and G bands of the carbon products, re-
spectively. The intensity ratio of the D to G bands (ID/IG) is gen-
erally accepted to reflect the degree of graphitization of carbo-
naceous materials and defect density. After Ag@CoNiFe was
loaded, the ID/IG of GO is increased from 1.03 to 2.48. The rela-
tive changes in ID/IG confirm the reduction of GO during the
in situ fabrication. The FTIR spectra of GO and graphene-sup-
ported Ag@CoNiFe core–shell NPs are shown in Figure 5. The
disappearance of the C=O peak at n˜ =1724 cmÀ1 and the CÀO
peak at n˜ =1052 cmÀ1 of GO after the formation of the gra-
phene-supported core–shell NPs can be seen clearly, which fur-
ther indicates that the GO is reduced to graphene during the
process.
Catalytic activities for the hydrolysis of AB and MeAB by
Ag@CoNiFe/graphene NPs
The as-synthesized Ag@CoNiFe/graphene NPs reduced by
MeAB with different compositions were tested for the hydroly-
sis of AB. Without the addition of Ag, the precursor Co, Ni, and
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Figure 5. FTIR spectra of GO and Ag@CoNiFe/graphene NPs.
Fe cannot be reduced by MeAB. In our study, the molar ratio
of Co and Ni was kept at 1. The Ag0.1@[(Co0.5Ni0.5)1ÀxFex]0.9 NPs
demonstrate different catalytic activities if the ratio of (Co+Ni)
to Fe is changed (Figure 6a). The best ratio of (Co+Ni) and Fe
in Ag0.1@[(Co0.5Ni0.5)1ÀxFex)0.9 is 9:1, and AB cannot be catalytical-
ly decomposed completely. However, with a ratio of (Co+Ni) to
Fe of 9:1, the effects of the Ag ratio were studied by changing
the
y value from 0.1 to 0.9. The best Ag ratio in
Agy@(Co0.5Ni0.5Fe0.1)1Ày is 0.1. Unexpectedly, as the molar ratio
of Ag (y value) increases from 0.1 to 0.9, the catalytic activities
of the as-synthesized NPs decrease gradually. The activity in
Figure 6. a) Hydrolysis of AB catalyzed by Ag0.1@[(Co0.5Ni0.5
(x=0.1–0.9) NPs, A: Ag0.1[(Co0.5Ni0.5 0.1Fe0.9 0.9, B: Ag0.1[(Co0.5Ni0.5
0.9, D: Ag0.1[(Co0.5Ni0.5 0.7Fe0.3 0.9, E:
0.9. b) Hydrolysis of AB catalyzed by
)
1ÀxFex]0.9/graphene
)
]
)
)
0.3Fe0.7 0.9, C:
]
Ag0.1[(Co0.5Ni0.5
Ag0.1[(Co0.5Ni0.5
)
)
0.5Fe0.5
0.9Fe0.1
]
]
]
terms of turnover frequency (TOF) is 118.5 molH minÀ1 molAg
À1
2
Agy@(Co0.5Ni0.5Fe0.1
B: Ag0.3[(Co0.5Ni0.5 0.9Fe0.1
Ag0.7[(Co0.5Ni0.5 0.9Fe0.1 0.3, E: Ag0.9[(Co0.5Ni0.5
)1Ày/graphene (y=0.1–0.9) NPs, A: Ag0.1[(Co0.5Ni0.5)0.9Fe0.1] ,
0.9
for the as-synthesized Ag0.1[(Co0.5Ni0.5)0.9Fe0.1]0.9/graphene NPs,
)
]
0.7, C: Ag0.5[(Co0.5Ni0.5
)
0.9Fe0.1
]
0.5, D:
which is higher than that of trimetallic and bimetallic core–
shell Ag-based NPs and most reported noble-metal-based NPs
if the TOF is normalized in terms of moles of noble metal
(Table 1).
)
]
)
0.9Fe0.1
]
0.1, catalyst/AB=0.05.
of the metal NPs over graphene by changing the reducing
agents, which further affects their catalytic activities. Addition-
ally, to study the effects of the supported materials on the cat-
For comparison, Ag@CoNiFe/graphene NPs generated by AB
as the reducing agent, and AgCoNiFe/graphene alloy NPs re-
duced by NaBH4 were also pre-
pared and applied to the catalyt-
ic hydrolysis of AB. It has been
reported that the catalytic activi-
ty increased if the size of the
metal NPs was decreased be-
cause of the higher surface areas
of the smaller metal NPs.[20]
Ag0.1[(Co0.5Ni0.5)0.9Fe0.1]0.9/gra-
Table 1. Catalytic activity of different noble-metal-based catalysts used for the hydrolytic dehydrogenation of
AB. (PSSA-co-MA=poly(4-styrenesulfonic acid-co-maleic acid), RGO=reduced graphene oxide)
Catalyst
TOF
TOF
Ea
Ref.
[molH molMÀ1 minÀ1
]
[molH molmetalÀ1 minÀ1
]
[kJmolÀ1
]
2
2
M=Ru, Pd, Ag
Ni0.74Ru0.26 alloy NPs
PSSA-co-MA-stabilized Ru nanoclusters 187.6
194.8
50.65
187.6
11.85
30.56
10.64
10.24
7.70
44
54
36.63
–
36.15
20.03
49.56
48Æ2
47
[19e]
[31]
This study
[32]
[23]
[22]
[22]
[33]
[34]
[35]
Ag@CoNiFe/graphene
Ni@Ru
Ag@CoNi/graphene
Ag@Co/graphene
Ag@Ni/graphene
Ru@Al2O3
RuCo (1:1)/g-Al2O3
Ru/g-Al2O3
PSSA-co-MA-stabilized Pd nanoclusters 19.9
118.5
114
106.4
102.4
77.0
39.6
32.9
23.05
phene NPs reduced by MeAB
with the smallest size exhibit the
highest catalytic activity toward
hydrolysis of AB, followed by the
core–shell NPs reduced by AB,
and the alloy NPs reduced by
NaBH4 exhibit the lowest activity
(Figure 7). These results indicate
that it is possible to achieve
much more control over the nu-
cleation and growth processes
39.6
16.45
23.05
19.9
67
44
[31]
RuCu (1:1)/g-Al2O3
Ag@C@Co
RGO/Pd
16.4
8.93
6.25
5.32
8.2
8.93
6.25
5.32
52
–
51
38.91
[34]
[35]
[37]
[30]
Ag/C/Ni
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Figure 7. Time plots of catalytic dehydrogenation of AB by
Ag0.1@[(Co0.5Ni0.5)0.9Fe0.1]0.9/graphene NPs reduced by MeAB and AB, respec-
tively, and Ag0.1@[(Co0.5Ni0.5)0.9Fe0.1]0.9/graphene reduced by NaBH4, catalyst/
AB=0.05.
alytic performances of the as-synthesized core–shell NPs, Ag@-
CoNiFe/C, Ag@CoNiFe/SiO2, and Ag@CoNiFe/g-Al2O3 NPs were
prepared and their catalytic activities towards the hydrolysis of
AB ware studied. Their catalytic activities are all inferior to that
of Ag@CoNiFe/graphene NPs (Figure 8), which highlights the
dominant factor of graphene to facilitate the hydrolysis of AB
in our system.
As the Ag0.1[(Co0.5Ni0.5)0.9Fe0.1]0.9/graphene catalyst reduced by
MeAB exhibits the highest activity, we used it for the further ki-
netic study. The plots of hydrogen generation from the hydrol-
ysis of AB and MeAB solution in the presence of different Ag@-
CoNiFe/graphene NP concentrations at (25Æ0.2)8C are shown
in Figures 9 and 10, respectively. The initial rate of hydrogen
generation was determined from the nearly linear initial por-
tion of each plot. The line slope of the plot of hydrogen evolu-
tion rate versus catalyst concentration on a log–log scale is
1.06 and 1.09 for AB and MeAB, respectively, which indicates
that the hydrolysis of AB and MeAB catalyzed by Ag@CoNiFe/
graphene NPs is first order with respect to the catalyst concen-
tration.
Figure 9. a) Plots of hydrogen evolution vs. time for the Ag@CoNiFe/gra-
phene-catalyzed hydrolysis of AB at different catalyst concentrations; b) plot
of hydrogen generation rate vs. the concentration of Ag@CoNiFe/graphene
(both in logarithmic scale).
To obtain the activation energy (Ea) of the hydrolysis of AB
and MeAB catalyzed by Ag@CoNiFe/graphene NPs, hydrolytic
reactions were performed in a temperature range of 25–408C.
The values of the rate constant k at different temperatures
were calculated from the slope of the linear part of each plot
shown in Figures 11a and 12a. The Arrhenius plot of ln k vs.
1/T for the catalyst is plotted in Figures 11b and 12b for the
hydrolysis of AB and MeAB, respectively, from which the appar-
ent Ea values of hydrolysis was determined to be approximate-
ly 36.63 and 35.33 kJmolÀ1, respectively, which are lower than
most of the reported Ea values (Table 1) and indicates the supe-
rior catalytic performance of the as-synthesized Ag@CoNiFe/
graphene NPs.
Recyclability
The reusability of a catalyst is crucial in practical applications.
The recyclability of Ag0.1@[Co0.5Ni0.5]0.9Fe0.1)0.9/graphene NPs up
to the fifth run for the hydrolysis of AB and MeAB is shown in
Figure 13.
The as-prepared catalysts retain 69.3 and 80.3% of their ini-
tial catalytic activities in the hydrolysis of AB and MeAB, re-
spectively, in the fifth run (Figure 13). The TEM image of Ag@-
CoNiFe/graphene NPs after the fifth run of the durability test is
Figure 8. Time plots of catalytic dehydrogenation of AB by Ag@CoNiFe NPs
reduced by MeAB with different supported materials, catalyst/AB=0.05.
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Figure 11. a) Plots of hydrogen evolution vs. time for the Ag@CoNiFe/gra-
phene-catalyzed hydrolysis of AB at four different temperatures in the range
of 25–408C, catalyst/AB=0.05; b) Arrhenius plot obtained from the data
shown in a). Figure 11a.
Figure 10. a) Plots of hydrogen evolution vs. time for the Ag@CoNiFe/gra-
phene-catalyzed hydrolysis of MeAB at different catalyst concentrations;
b) plot of hydrogen generation rate vs. the concentration of Ag@CoNiFe/
graphene (both in logarithmic scale).
tion during the hydrolysis of AB or MeAB should also be taken
into account. Moreover, from Figure 13, we can see that the
in situ synthesized Ag@CoNiFe/graphene NPs are magnetic
and thus can be separated from the reaction solution by an ex-
ternal magnet, which makes the practical recycling of the NPs
more convenient.
shown in Figure 1g. As can be seen clearly from the TEM
image, there is no noticeable change in the morphology of the
as-synthesized NPs on graphene. These results indicate that
graphene can stabilize the as-synthesized NPs at least for five
cycles. The X-ray photoelectron spectra (XPS) of the as-synthe-
sized Ag@CoNiFe/graphene catalysts before and after five
cycles are shown in Figure 14. The XPS Ag3d peak of the cata-
lyst is shown in Figure 14a and b. Two peaks can be seen at
a binding energy (BE) of 372.4 and 366.6 eV. This is in good
agreement with the values for zero-valent Ag, which corre-
sponds to Ag3d5/2 and Ag3d3/2. The peaks of Co2p are shown
in Figure 14c and d. There are two peaks at BE=779.5 and
776.7 eV, which correspond to oxidized Co and zero-valent Co,
respectively. The peaks of Ni2p are shown in Figure 14e and f;
the two peaks at BE=854.5 and 851.0 eV represent oxidized Ni
and zero-valent Ni. The peaks of Fe2p are shown in Figure 14g
and h, in which the peak at BE=711.4 eV represents oxidized
Fe. These results indicate that the core Ag metal is stable,
whereas the outside shell of Co, Ni, and Fe are oxidized to
varying degrees maybe during the sample preparation[38] and
during the catalytic process.[23] Therefore, the decrease in the
catalytic activity may be because of the oxidation of the shell
metals. Meanwhile, the increased viscosity of the solution and
the deactivation effect of the increasing metaborate concentra-
Conclusions
We have developed a facile in situ one-step method for the
synthesis of graphene-supported tetrametallic core–shell mag-
netic Ag@CoNiFe nanoparticles (NPs). Their catalytic activities
towards the hydrolytic dehydrogenation of AB and MeAB
under ambient conditions were also studied. Compared with
those reduced by AB and NaBH4, the as-synthesized Ag@Co-
NiFe/graphene catalysts reduced by MeAB exert the highest
catalytic activity with a turnover frequency (TOF) value of
118.5 molH minÀ1 molAg
.
Furthermore, the as-synthesized
À1
2
Ag@CoNiFe/graphene NPs showed a good durability, stability,
and magnetic recyclability for the hydrolytic dehydrogenation
of AB and MeAB, which makes the practical recycling of the
catalyst convenient. Moreover, this simple synthetic method
can be extended to other mutilmetallic core–shell systems for
more applications.
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Figure 12. a) Plots of hydrogen evolution vs. time for the Ag@CoNiFe/gra-
phene-catalyzed hydrolysis of MeAB at four different temperatures in the
range of 25–408C, catalyst/MeAB=0.05; b) Arrhenius plot obtained from the
data shown in a).
Figure 13. a) and b) Hydrogen generation from AB and MeAB catalyzed by
Ag@CoNiFe/graphene NPs from the first to fifth cycles, catalyst/AB=0.05;
b) inset: photographs of the Ag@CoNiFe/graphene NPs before (left) and
after (right) magnetic separation.
Experimental Section
Graphene oxide (GO) preparation
Materials
GO was synthesized according to the literature.[39] In an improved
synthesis of GO, a 9:1 mixture of concentrated H2SO4/H3PO4
(360:40 mL) was added to a mixture of graphite flakes (3.0 g) and
KMnO4 (18.0 g). The mixture was then heated to 508C and stirred
for 12 h. The reaction was cooled to RT and poured onto ice
(~400 mL) with 30% H2O2 (3 mL). The addition of portions of H2O2
(2 mL) was continued until the observation of a permanent yellow
color, which indicated the complete oxidation of graphite. The re-
sultant solution was centrifuged to obtain the product. The prod-
uct was washed with deionized water, 30% diluted hydrochloric
acid, and absolute ethyl alcohol many times and dried under
vacuum at 258C.
Ammonia borane (AB, Aldrich, 90%), sodium borohydride (NaBH4,
Sinopharm Chemical Reagent Co., Ltd., ꢀ96%), methylamine hy-
drochloride (CH3NH2·HCl, Sinopharm Chemical Reagent Co., Ltd.,
ꢀ96%), tetrahydrofuran (C4H8O, Sinopharm Chemical Reagent Co.,
Ltd., ꢀ99%), dimethyl ether anhydrous (C4H10O, Sinopharm Chemi-
cal Reagent Co., Ltd., ꢀ99.7%), silver nitrate (AgNO3, AR), cobalt
chloride hexahydrate (CoCl2·6H2O, Sinopharm Chemical Reagent
Co., Ltd., ꢀ99%), nickel(II) chloride hexahydrate (NiCl2·6H2O), fer-
rous chloride tetrahydrate (FeCl2·4H2O, Sinopharm Chemical Re-
agent Co., Ltd., ꢀ99%), graphite power (Sinopharm Chemical Re-
agent Co., Ltd., ꢀ99.85%), potassium permanganate (KMnO4,
Shanghai Chemic Co., Ltd., ꢀ99.5%), hydrogen peroxide (H2O2, Si-
nopharm Chemical Reagent Co., Ltd., ꢀ30%), phosphoric acid
(H3PO4, Sinopharm Chemical Reagent Co., Ltd., AR), sulfuric acid
(H2SO4, Sinopharm Chemical Reagent Co., Ltd., 95~98%), Ketjen
black EC-300 J (Triquo Chemical Co., Ltd.), neutral silica power
(SiO2, Branch of Qingdao Haiyang Chemical Co., Ltd.), and alumi-
num oxide neutral (g-Al2O3, Sinopharm Chemical Reagent Co., Ltd.,
FCP) were used as obtained. We used ordinary distilled water as
the reaction solvent.
Preparation of methylamine borane (MeAB)
MeAB was synthesized according to the method reported in the lit-
erature.[40] Sodium borohydride (3.783 g, 0.1 mol) and methylamine
hydrochloride (6.752 g, 0.1 mol) were added to a 500 mL two-
necked round-bottomed flask equipped with a condenser. THF
(200 mL) was transferred into the flask with vigorously stirring. The
reaction was performed at RT under a nitrogen atmosphere. After
12 h, the resultant solution was filtered by suction filtration, and
the filtrate was concentrated under vacuum at RT. The product was
purified by ether.
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above. If the molar ratio of (Co2++Ni2+)/Fe2+ was kept constant at
1, the molar ratio of Ag+/(Ag++(Co2++Ni2+)+Fe2+) was changed to
several values (0.1, 0.3, 0.5, 0.7, 0.9). If the molar ratio of
Ag+/(Ag++(Co2++Ni2+)+Fe2+
) was kept as 0.1, the value of
(Co2++Ni2+)/Fe2+ was changed from 0.1 to 0.9 (0.1, 0.3, 0.5, 0.7,
0.9).
Catalytic hydrolysis of AB and MeAB
Sets of experiments with different concentrations of
Ag0.1[(Co0.5Ni0.5)0.9Fe0.1]0.9/graphene (0.02, 0.03, 0.04, 0.05 mmol)
were performed at RT, and the AB concentration was kept the
same (1 mmol) to determine the rate law of the catalytic hydrolysis
of AB. For MeAB, sets of experiments with different concentrations
of Ag0.1[(Co0.5Ni0.5)0.9Fe0.1]0.9/graphene (0.03, 0.035, 0.04, 0.05 mmol)
were performed at RT. The temperature was varied at (25Æ0.2),
(30Æ0.2), (35Æ0.2), and (40Æ0.2)8C, and the concentration of
Ag0.1[(Co0.5Ni0.5)0.9Fe0.1]0.9/graphene (0.05 mmol) and AB (or MeAB)
(1 mmol) were kept constant to obtain the activation energy.
Stability test for the hydrolysis of AB and MeAB
A solution of MeAB (1.9 mL, 90.0 mg, 2 mmol) was added to cobalt
chloride (0.9 mL), nickel chloride (0.9 mL), and ferrous chloride so-
lution (0.2 mL; 0.0225 molLÀ1), silver nitrate solution (0.1 mL,
0.05 molLÀ1), and GO (10 mg). The evolution of gas was monitored
as described above. After the hydrogen generation reaction was
completed, new aqueous AB or MeAB solution (1 mmol, 2 mL) was
added into the reaction flask. The evolution of gas was monitored
again by using the gas burette. Such tests of the catalyst for the
hydrolysis of AB and MeAB were performed five times in air.
Different supports
Sets of experiments with different supports (such as SiO2, g-Al2O3,
and carbon black) were performed at RT. All the experiments were
performed as described above.
Figure 14. XPS spectra of Ag@CoNiFe/graphene NPs before (left) and after
(right) five cycles.
Characterization
In Situ synthesis of Ag@CoNiFe/graphene catalysts
TEM images were recorded by using a FEI Tecnai G20 TEM instru-
ment operating at 200 kV. Powder XRD patterns were measured by
using a Bruker D8-Advance X-ray diffractometer using a CuKa radia-
tion source (l=0.154178 nm) with a velocity of 68minÀ1. FTIR spec-
tra were collected at RT by using a Thermo FTIR-iS10 instrument
using KBr discs in the 400–4000 cmÀ1 region. Raman spectra were
measured by using a confocal Raman microscope (Renishaw, RM-
1000) at 514.5 nm excitation. XPS measurements were performed
by using a Kratos XSAM 800 spectrophotometer. UV/Vis spectra
were recorded by using a Persee TU-1810 spectrophotometer.
In a typical experiment, GO (10 mg) was put into a 25 mL two-
necked round-bottomed flask, and cobalt chloride (0.9 mL), nickel
chloride (0.9 mL), and ferrous chloride (0.2 mL) aqueous solutions
(0.0225 molLÀ1), and silver nitrate aqueous solution (0.1 mL;
0.05 molLÀ1) were added. One neck was connected to a gas bu-
rette, and the other was connected to a pressure equalizer, which
used to introduce MeAB. An aqueous solution (1.9 mL) that con-
tained MeAB (90.0 mg, 2 mmol) was kept in the pressure equalizer
(90.0 mg). The reaction was started when the MeAB solution was
added to the flask with vigorous stirring, and the evolution of the
gas was monitored by the gas burette. When the hydrogen gener-
ation reaction was completed, aqueous solution (2 mL) that con-
tained AB (1 mmol) was added to the flask, and the evolution of
the gas was monitored. A water bath was used to control the tem-
perature of the reaction solution. For comparison, the Ag@CoNiFe
catalyst without GO reduced by MeAB, Ag@CoNiFe/graphene re-
duced by AB, and AgCoNiFe/graphene reduced by NaBH4 were
synthesized by a similar method.
Acknowledgements
The authors thank the reviewers for valuable suggestions. This
work was financially supported by the National Natural Science
Foundation of China (21201134), the Natural Science Foundation
of Jiangsu Province (BK20130370), the Natural Science Founda-
tion of Hubei Province (2013CFB288), and the Large-scale Instru-
ment and Equipment Sharing Foundation of Wuhan University.
Other Ag@CoNiFe NPs with different Ag, (Co+Ni), and Fe composi-
tions were synthesized by a similar method to that described
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Received: January 16, 2014
Revised: March 19, 2014
Published online on && &&, 0000
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L. Yang, J. Su, W. Luo,* G. Cheng
Core–shell casts a spell: Well-dispersed
tetrametallic core–shell Ag@CoNiFe/gra-
phene nanoparticles exert a satisfactory
catalytic activity and recyclability to-
wards the hydrolysis of ammonia
&& – &&
Size-Controlled Synthesis of
Tetrametallic Ag@CoNiFe Core–Shell
Nanoparticles Supported on
Graphene: A Highly Efficient Catalyst
for the Hydrolytic Dehydrogenation of
Amine Boranes
borane and methylamine borane.
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