RuCo NPs supported on MIL-96(Al) as highly active catalysts for the hydrolysis of ammonia borane

Article abstract of DOI:10.1016/j.jallcom.2016.10.055

Bimetallic RuCo nanoparticles (NPs) were successfully deposited on the highly porous and hydrothermally stable nanofibrous metal-organic framework MIL-96(Al) by using a simple liquid impregnation strategy, and the powder XRD, FT-IR, BET, TEM, EDX, ICP-AES

Full text of DOI:10.1016/j.jallcom.2016.10.055

Accepted Manuscript
RuCo NPs supported on MIL-96(Al) as highly active catalysts for the hydrolysis of
ammonia borane
Di Lu, Guofeng Yu, Yue Li, Menghuan Chen, Yaxi Pan, Liqun Zhou, Kunzhou Yang,
Xing Xiong, Peng Wu, Qinghua Xia
PII:
S0925-8388(16)33163-2
10.1016/j.jallcom.2016.10.055
JALCOM 39222
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 25 June 2016
Revised Date: 29 September 2016
Accepted Date: 6 October 2016
Please cite this article as: D. Lu, G. Yu, Y. Li, M. Chen, Y. Pan, L. Zhou, K. Yang, X. Xiong, P. Wu, Q.
Xia, RuCo NPs supported on MIL-96(Al) as highly active catalysts for the hydrolysis of ammonia borane,
Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.10.055.
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Graphical abstract:
Hydrogen generation rate from the hydrolysis of AB: different Ru, Co based catalysts.

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RuCo NPs supported on MIL-96(Al) as highly active
catalysts for the hydrolysis of ammonia borane
Di Lua, Guofeng Yub, Yue Lia, Menghuan Chena, Yaxi Pana, Liqun Zhoua, , Kunzhou
Yanga, Xing Xionga, Peng Wua, Qinghua Xiaa
aHubei Collaborative Innovation Center for Advanced Organic Chemical Materials,
Ministry of Education Key Laboratory for the Synthesis and Application of Organic
Functional Molecules, College of Chemistry and Chemical Engineering, Hubei
University, Wuhan 430062, PR China
bHubei Engineering University, Xiaogan 432000, PR China
Abstract: Bimetallic RuCo nanoparticles (NPs) were successfully deposited on the
highly porous and hydrothermally stable nanofibrous metal-organic framework
MIL-96(Al) by using a simple liquid impregnation strategy, and the powder XRD,
FT-IR, BET, TEM, EDX, ICP-AES and XPS were employed to characterize the
structure, size, composition and loading metal electronic states of the RuCo@MIL-96
catalysts. The catalytic property of RuCo@MIL-96 for hydrogen generation from the
hydrolysis of ammonia borane (AB) at room temperature was investigated. The
results show that Ru1Co1@MIL-96 exhibits much enhanced catalytic activity
compared with monometallic Ru, Co counterparts loadings and RuCo NPs due to the
uniform distribution of metal NPs and synergetic effect between Ru and Co particles
as well as bi-functional effects between RuCo NPs and the host of MIL-96. The turn
over frequency (TOF) value of the Ru1Co1@MIL-96 catalyst is determined to be
320.7 mol H2 min-1 (mol Ru)-1 , which is higher than most of the reported TOF values
for the same reaction, and the activation energy (Ea) is 36.0 kJ mol-1. Moreover, this
catalyst exhibits satisfied durability after five cycles for the hydrolytic
Corresponding authors.
E-mail addresses:ldff2016@163.com (D. Lu), zhoulq2003@163.com (L. Zhou).

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dehydrogenation of ammonia borane.
Keywords: MIL-96
RuCo loading catalysts
Ammonia borane
Hydrogen
storage
1.Introduction
Hydrogen, producing only water as a byproduct, has been considered to be the
most promising carrier for alternative energy applications [1]. But for the
development of hydrogen energy economy, searching for safe and efficient hydrogen
storage materials remains one of the most challenging barriers [2]. Various hydrogen
storage approaches are currently being investigated, including metal hydrides [3],
sorbent materials [4], and chemical hydride systems [5]. Among the chemical
hydrides, ammonia borane (NH3BH3, denoted as AB) appears to be an appropriate
hydrogen storage material because of its high hydrogen content of 19.6wt%, excellent
stability under ambient conditions, and environmental benign [6]. The release of
hydrogen from AB can be obtained through thermal dehydrogenation in the solid state
[7], hydrolysis [8] or methanolysis in solution [9]. With an appropriate catalyst, the
hydrolysis of AB can release as much as 3 mol of hydrogen per mol of AB. The
catalytic hydrolysis reaction can be briefly expressed as follows [10]:
NH 3 BH3 + 2H 2O catalyst → NH 4 BO2 + 3H 2
Heterogeneous catalysts containing noble metal NPs exhibited good catalytic
property toward AB hydrolysis, with ruthenium NPs considered as one of the most
effective catalysts in the system [11]. However, the widespread utilization of noble
metal-based catalysts was hindered by their high price and limited abundance.
Bimetallic NPs consisting of noble metal and inexpensive first-row transition metal
have exhibited good catalytic performance in many fields mainly due to the
synergistic interactions between two or three different metals [12-14]. On the other
hand, due to the high specific surface area and tunable pore size, metal-organic
frameworks (MOFs) have attracted growing attention in the application of gas
sorption and storage [15], drug delivery [16], and molecular separation [17]. The

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metal NPs anchored by MOFs are expected to improve the catalytic activity and
stability, since MOFs with confined pores and cavities could control the growth and
dispersion of metal NPs and restrain the migration and aggregation of metal NPs. Up
to now, a series of metal NPs@MOFs catalysts were tested for hydrolytic
dehydrogenation of AB, such as Ru@MIL-101 [18], Pd@MIL-101 [19], Ru@
MIL-96(Al) [20], RhNi@ ZIF-8 [21], RuCuCo@MIL-101 [14], CuCo@MIL-101 [22]
and so on. Therefore, developing a facile one-step route to construct the bimetallic or
trimetallic NPs in MIL-96 with high catalytic activities is highly desirable.
Herein, for the first time, the catalyst of bimetallic RuCo NPs immobilized in
MIL-96(Al) has been successfully synthesized. For comparison with the RuCo@
MIL-96 catalytic activities for hydrolysis of AB at room temperature, monometallic
Ru, Co counterparts loadings and RuCo NPs were also prepared. The structure,
morphology, size and composition of the catalysts with different metal NPs loadings
were discussed in detail, and then investigated catalytic activity, durable stability of
various catalysts and the influence factors. In addition, the activation energy (Ea) for
the hydrolysis reaction has been measured by different temperature experiment.
2. Material and methods
2.1. Reagents and Chemicals
Aluminum nitrate nonahydrate, 1,3,5-benzenetricarboxylic acid (H3btc), N,N-
dimethylformamide (DMF), ruthenium chloride hydrateꢀcobalt chloride hexahydrate,
sodium borohydride were purchased from Sinopharm Chemical Reagent Co., Ltd..
Ammonia borane (AB) was obtained from Energy Chemical. All chemicals employed
were of analytical grade and used without further purification. Additionally, we used
de-ionized water as the reaction solvent.
2.2. Synthesis of MIL-96(Al)
MIL-96(Al) was hydrothermally synthesized to follow the reported method [23].
Typically, H3btc (208 mg), Al(NO3)3·9H2O (460 mg) and de-ionized water (7 mL)
were placed in a 40 mL Teflon-liner autoclave and heated at 210 ℃ for 12 h. After
natural cooling, the suspension was filtered off to separate the white powder of

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MIL-96(Al) with formula Al12O(OH)18(H2O)3(Al2(OH)4)[btc]6·24H2O and then
further purified by solvothermal treatment in DMF at 150
for 5 h. The separated
resulting solid was refluxed overnight in a suspension of water at 100 for 12 h and
immediately filtered. The resulting white solid was finally dried overnight in the
vacuum oven at 80 for further use.
The structure description of MIL-96(Al) phase are learned from published papers
[23], which exhibits a 3D framework resulting from the connection of aluminium
octameric clusters linked through the trimesate (btc) ligand. An inorganic part consists
of a 2D network containing chains of aluminum AlO2(OH)4 and AlO4(OH)2 octahedra,
interconnected with each other to generate a hexagonal 18-membered ring in the (a,b)
plane have been found in the MIL-96 framework (Fig. 1). The cohesion of the
structure is ensured by the connection of the oxo-centered trinuclear aluminum units
with the 2D network of aluminum octahedra through the trimesate molecules. The
structure fully proves that MIL-96 has the characteristic of porous, which provides the
possibility for the load of RuCo NPs.
2.3. Synthesis of Ru1Co1@MIL-96 and other catalysts
The Ru1Co1@MIL-96 catalyst was synthesized as follows: 200 mg of MIL-96
(Al) was added to a flask containing 30 mL of de-ionized water. Ultrasonication was
required to get a uniform dispersion. Next, 10 mL RuCl3 solution (0.01 mol·L-1 ) and
10 mL CoCl2·6H2O solution (0.01 mol·L-1 ) was added into the flask under magnetic
stirring for 5 h. Then, 10 mL aqueous solution of NaBH4 (50 mg) was added to this
mixture and the resulting solution was stirred at room temperature for 5 h. The
product was collected through filtration and dried in the vacuum oven at 80 for 12h
to obtain the Ru1Co1@MIL-96 catalyst.
In order to compare the catalytic activity, the preparations of the Ru1Co0.5@
MIL-96, Ru1Co2@MIL-96 and Ru1Co3@MIL-96 are following the above analogous
process. During the synthesis of mono-metal Ru@MIL-96, Co@MIL-96 and pure
RuCo NPs catalysts, the amounts of Ru and Co were consistent with that of Ru1Co1@
MIL-96. In a typical synthesis of Co@MIL-96, 200 mg MIL-96 and 10 mL 0.01 mol

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L-1 CoCl2·6H2O were diluted in 30 mL of de-ionized water. The solution was then
introduced to a flask which contained 50 mg of NaBH4 with vigorous stirring. And
RuCo NPs was prepared as follows: 0.25 mL 0.01 mol·L-1 RuCl3 and 0.25 mL 0.01
mol·L-1 CoCl2 solution were added to a flask containing 30 mL of de-ionized water.
Ultrasonication was required for 20 min to get a uniform dispersion, and then an
aqueous solution of 10 mL 0.013 mol·L-1 NaBH4 was added to this mixture followed
by magnetic stirring for 5 h. The product was collected through filtration, washed
with de-ionized water and ethanol for three times, and dried at 80
vacuum oven for further use.
overnight in a
2.4. Hydrolytic dehydrogenation of ammonia borane
The catalytic activity of RuCo@MIL-96 in the hydrolysis of AB was determined
by measuring the rate of hydrogen generation. The experiment was conducted in a
two-necked round bottom flask contain a mixture of 5 mg RuCo@MIL-96 and 10 mL
de-ionized water. One neck was connected to a pressure-equalization funnel to
introduce 10 mL aqueous solution of NH3BH3 (18.5 mg, 0.6 mmol) and the other neck
was connected to a gas burette to monitor the volume of the gas evolution. The
reaction started when the aqueous solution was added to the catalyst, and the volume
of the generated hydrogen gas was recorded by the displacement level of water in the
burette at a certain time interval (30 s). A water bath was used to control the reaction
temperature at 25 . In different Ru/Co molar ratio composites, 5 mg of Ru1Cox
@MIL-96 (x=0, 0.5, 1, 2, 3) catalysts were employed for the hydrolysis of ammonia
borane.
In order to get the activation energy (Ea) of the hydrolytic reaction, 18.5 mg AB
was hydrolyzed with 5mg Ru1Co1@MIL-96 at different temperatures (25, 30, 35 and
40 ꢁ).
The durability of Ru1Co1@MIL-96 catalyst in the hydrolysis of AB was
evaluated by performing the recyclability tests at room temperature. In a typical test,
10 mL of Ru1Co1@MIL-96 (5 mg) suspension solution and 18.5 mg AB (10 ml, 0.6
mmol) were added into the reaction flask at 25℃, and the evolution of gas was

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measured as described above. After the hydrolysis reaction was completed, another
equivalent of AB (18.5mg) was subsequently added into the reaction flask. The
evolution of gas was monitored by using the gas burette. The tests were repeated five
times at room temperature. After the tests, the catalysts were separated from the
reaction solution by centrifugation and washed with water for several times and dried
in the vacuum oven at 80 .
In a typical reusability test, 18.5 mg AB (10 mL, 0.6 mmol) was added to 10 mL
of Ru1Co1@MIL-96 (5 mg) suspension solution, the evolution of gas was monitored
as described above. After the hydrolysis reaction was completed, the
Ru1Co1@MIL-96 catalyst was isolated and redispersed in 10 mL de-ionized water.
Then, 18.5 mg AB (10 mL, 0.6 mmol) was added into the reaction flask for a
subsequently run of hydrolysis at 25 .
The maximum productivity of Ru1Co1@MIL-96 catalyst in the hydrolysis of AB
was determined by measuring the turnover number (TON, moles of H2/moles of
catalyst). In a typical test, 10 mL of Ru1Co1@MIL-96 (5 mg) suspension solution and
10 mL AB (18.5 mg) were added into the reaction flask at 25 , and the evolution of
gas was measured as described above. Once complete conversion is achieved, another
equivalent of AB was added and the reaction was continued until no hydrogen gas
evolution was observed.
2.5.Catalyst characterization
The morphologies and composition of the samples were observed by using a
Tecnai G20 U-Twin transmission electron microscope (TEM) equipped with an
energy dispersive X-ray detector (EDX) at an acceleration voltage of 200 kV. Powder
X-ray diffraction (XRD) patterns were measured by a Bruker D8-Advance X-ray
diffractometer using Cu Kα radiation source (λ=0.154178 nm) with a velocity of 10°
min-1 to identify the phase purity and crystallinity. Fourier transform infrared (FT-IR)
spectroscopic experiment was carried out on a Nicolet (Madison, WI, USA) Impact
420 apparatus to determine the structure of MIL-96. The electronic states of the
as-synthesized samples were investigated by X-ray photoelectron spectroscopy (XPS,

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Thermo ESCALAB 250Xi). The metal loadings of catalysts were determined by an
IRIS Intrepid IIXSP inductively coupling plasma-atomic emission spectrometer (ICP-
AES). The surface area measurements were performed with N2 adsorption-desorption
isotherms obtained at 77 K using a Micromeritics ASAP 2020 instrument.
3. Results and discussion
The XRD patterns of the as-prepared MIL-96, Ru@MIL-96, Co@MIL-96,
Ru1Co1@MIL-96 and Ru1Co1@MIL-96 after the fifth run catalysts are illustrated in
Fig. 2A, respectively. It can be seen that the XRD pattern of MIL-96 is in good
agreement with the previously reported one [23], confirming that MIL-96 is
successfully synthesized. The patterns of Ru@ MIL-96, Co@MIL-96 and
Ru1Co1@MIL-96 samples are matched with those of the host MIL-96, indicating that
the integrity of the MIL-96 framework is maintained well during the catalyst
preparation and catalytic process. However, the decrease in intensity of peaks as
compared with MIL-96 can be explained by the changes in the charge distribution and
electrostatic fields as a result of the existence of RuCo NPs on the surface and
interaction of their electrophilic surface with framework atoms [24]. Additionally, the
diffraction peaks corresponding to the Ru and/or Co NPs were indicated in Fig. 2B,
the weak intensities were probably as a result of low metal loading or the amorphous
phases of Ru and Co NPs [25]. Moreover, Fig.2C shows the diffraction peaks of Ru at
around 44.1˚ in the Ru@MIL-96 catalyst. However, the peak of Ru shifts slightly to
higher 2θ values from 44.1˚ to 44.7˚ in Ru1Co1@MIL-96 compared to Ru@MIL-96,
that indicates the incorporation of small Co atoms into Ru lattice results in a lattice
contraction [26]. The above results show that RuCo alloys are formed in
Ru1Co1@MIL-96 catalyst rather than a physical mixture of individual metal NPs [27].
Based on the formation of RuCo alloy NPs, positive synergistic effect among noble
metal Ru and non-noble metal Co could be achieved, that leads to the highly catalytic
activity for catalytic hydrolysis of AB.
The X-ray photoelectron spectroscopy (XPS) was employed to demonstrate
metallic electronic states in the Ru1Co1@MIL-96 catalyst. Fig. 3a shows the survey

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scan X-ray photoelectron spectra of as-prepared Ru1Co1@MIL-96 catalyst. It can be
seen that the main absorptions belong to C 1s, Ru 3d, Ru 3p, Co 2p, and O 1s. Due to
the strong overlap of the C 1s and Ru 3d peaks around 284.7 eV, it is very difficult to
analyze this region for ruthenium properly. Fig. 3b shows the absorptions located at
462.0 and 485.1 eV for Ru 3p3/2 and Ru 3p1/2 , respectively, are readily assigned to the
Ru(0) [28] . In Fig. 3c, the peak of Co 2p reveals four peaks at 797.8 and 803.6 eV for
Co(II) 2p1/2 , 781.6 and 786.3 eV for Co(II) 2p3/2 [29-31]. The two weak peaks at
777.9 and 793.0 eV are attributed to Co(0) 2p3/2 and Co(0) 2p1/2 [22]. These results
indicate that Ru is stable in the catalyst, whereas Co may be oxidized to certain degree
during the synthesis of the Ru1Co1@MIL-96 catalyst.
The morphologies, particles dispersed state and element composition of RuCo
NPs loaded on MIL-96 were further characterized by TEM and EDX techniques. Fig.
4 shows the TEM images of the as-synthesized (a,b) and used (c,d) Ru1Co1@MIL-96,
in which the lattice fringes can be clearly observed and there are many small particles
loaded on the crystal surface. Fig. 4a, c and Fig. 4b, d are the low and high resolution
TEM images of the Ru1Co1@MIL-96 sample, respectively. From Fig. 4a,b, it can be
seen that the MIL-96 displays a nanofibrous morphology with a width of 5~10 nm
and length of 100~200 nm, and most RuCo NPs with a narrow particle size
distribution are uniformly dispersed on the surface of MIL-96. Particle size
distribution of Ru@MIL-96 and Co@MIL-96 were shown in Fig. 4g,h, it can be seen
that the mean particle diameter of Ru in the Ru@MIL-96 and Co in Co@MIL-96
were about 2.3±0.3nm and 6.7±0.3nm, respectively. The mean particle diameter of
RuCo particles of as-prepared Ru1Co1@ MIL-96 was about 1.7 nm (Fig. 4e), and the
standard deviation was determined to be 0.3nm, while the diameter of pore of the
support MIL-96 was about 1.66 nm, resulting in most of RuCo NPs locate at the
external surface of the crystal. However, it could not preclude that there are still some
smaller RuCo NPs (<1.66 nm) embedded within the frameworks, these NPs difficult
to be identified by TEM imaging. And no significant aggregate is formed, which is the
basis for high performance of the catalyst, probably because the MIL-96 structures
offer steric restrictions that strengthen the synergistic reaction between Co and Ru

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NPs on the surface of MIL-96. In Fig. 4(c,d), after five catalytic cycles, it can be seen
that the Ru1Co1@MIL-96 catalyst still remains the structure of the MIL-96 and shows
an excellent stability, and a number of NPs are well dispersed on the surface of
MIL-96. The particle size distribution of the Ru1Co1@MIL-96 catalyst after the fifth
run was shown in Fig. 4i, the mean particle diameter and the standard deviation of
RuCo particles of was about 1.9±0.3nm. From Fig. 4i, the particle size distribution of
Ru1Co1@MIL-96 after the fifth run almost keeps unchanged compared with that of
the as-prepared Ru1Co1@MIL-96. In order to further confirm the presence of RuCo
NPs, the EDX spectrum was measured, see Fig. 4f. Moreover, the actual RuCo
loading effect and ratio of the metals Ru, Co in RuCo@MIL-96 catalysts were
measured by ICP-AES, as shown in Table 1. The results show that the actual ratios of
Ru:Co is almost similar to the precursor ratios 1:0,1:0.5,1:1,1:2,1:3 in the Ru1Cox@
MIL-96 (x=0,0.5,1,2,3) catalysts. And the Co contents are obviously lower than its
initial additive amount in all samples. This is probably because Ru3+ can be readily
reduced to Ru by NaBH4 , but it is difficult to reduce Co2+ to Co due to the lower
reduction potentials of Co2+/Co (reduction potentials: Eθ(Ru3+/Ru)=+0.40 eV;
Eθ(Co2+/Co)= −0.28 eV). In the Ru1Co1@ MIL-96 catalyst, the actual loadings of Ru
and Co are 2.07 and 1.19 wt%, respectively.
From Fig. 5(a,b), the specific surface areas estimated from the N2 adsorption-
desorption isotherms of MIL-96(Al) support and the as-prepared Ru1Co1@MIL-96(Al)
were 443.6 m2/g and 205.4 m2/g, respectively, further confirming the highly porous
structure of MIL-96 support. Resulted from the porous structure, the surface area of
MIL-96(Al) is much higher than that of Ru1Co1@MIL-96, the significant decrease in
the surface area of Ru1Co1@MIL-96 indicates that the cavities of MIL-96(Al) are
either occupied or blocked by the well-dispersed RuCo NPs. Fig. 5(c) shows the pore
size of MIL-96 and Ru1Co1@MIL-96. The pore volume of MIL-96 was determined to
1
be 0.094 cm3 g− , and its pore size was calculated to be 1.66 nm. However, after the
loading of RuCo NPs, the pore volume of MIL-96 decreased to 0.051 cm3 g−1, and the
corresponding pore size from 1.66 to 1.08 nm. The reason for this phenomenon is that
the relatively small size RuCo alloy NPs occupied or blocked the cavities of MIL-96

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and the big size alloy NPs mainly deposited on the surface of MIL-96, that directly
lead to a decrease in the pore volume and pore size.
The functional groups of MIL-96 and MIL-96 supported RuCo NPs were
investigated by FT-IR, as shown in Fig. 6. It can be seen that the intensity and position
of FT-IR peaks of Ru1Co1@MIL-96 after the fifth run show no obvious change
compared to that of MIL-96 and initial Ru1Co1@MIL-96 catalyst, indicating that the
crystalline structure of the host of MIL-96 is unchanged, that provides favorable
conditions for RuCo NPs effectively immobilized in the framework of MIL-96 after
the catalytic cycles.
The catalytic properties of the as-obtained Ru@MIL-96, Co@MIL-96 and
Ru1Co1@MIL-96 samples were systematically evaluated in the catalytic
hydrogenation of AB at room temperature. Fig. 7 shows that the Ru1Co1@MIL-96
exhibits the highest activity compared with monometallic Ru@MIL-96 and Co@
MIL-96. Furthermore, the same amount of RuCo NPs reduced by NaBH4 without the
support MIL-96(Al) also is applied to hydrolysis of AB, only 31 mL H2 were released
for more than 15 min, and MIL-96 almost no activity. Reasonably, the Ru1Co1@
MIL-96 remarkably high catalytic activity is first attributed to the synergetic effect
between Ru and Co alloy NPs. For alloy NPs, synergetic effects may be mainly
ascribed to electronic effects, which are important when the metal atoms have
significant differences in electronegativity [32], and geometric effects. In the present
case, the observed synergetic effects in catalysis might be mainly due to the electronic
effects, not the geometric effects, because the disparity of electronegativitie, Ru (2.3)
and Co(1.88), is prominent. Additionally, the bi-functional effects between RuCo
alloy NPs and MIL-96 are significantly enhanced the reaction activity of Ru1Co1@
MIL-96 catalyst, in which MIL-96 can provide a good 3D framework for RuCo NPs
in spite of it does not involved in the hydrolysis reaction of AB. By calculating, the
turn over frequency (TOF) value of Ru1Co1@MIL-96 is determined to be 320.7 mol
H2 min-1 (mol Ru)-1 , which is higher than most Ru-based catalysts ever reported for
the catalytic hydrolysis of AB (Table 2).
For a comprehensive elucidation of the catalytic activity of the RuCo@MIL-96

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catalytic architecture towards hydrolytic dehydrogenation of AB, we followed a
systematic approach, where the single-component samples Ru@MIL-96 and Co@
MIL-96 catalytic performance were studied in addition to binary combinations of
these subcomponents, namely Ru1Co0.5@MIL-96, Ru1Co1@MIL-96, Ru1Co2@MIL
-96 and Ru1Co3@MIL-96 (Fig. 8a). Evidently, Ru1Co1@MIL-96 catalyst provides the
highest activity compared to all other investigated catalysts. By changing the molar
ratio of Ru to Co, the reaction rate increased at first and then decreases, as shown in
Fig. 8b. The observed “volcano”- type activity of the RuCo@MIL-96 catalysts versus
Ru/Co molar indicates that neither Ru nor Co has enough activity for catalyzing the
hydrolytic dehydrogenation of AB at complete conversion. This phenomenon may
relevant to catalytic mechanism of the RuCo@MIL-96 catalysts for the hydrolysis
reaction of AB. To the best of our knowledge, the heterogeneous catalytic reaction
takes place on the surface area of the metal catalyst [45]. The mechanism involves
two steps. Firstly, AB interacts with the surface of RuCo alloy NPs to form the
activated transient Metal-H, which is the prerequisite for the hydrolysis reaction. Then,
H2 was released after the attack of water on the Metal-H species [46,47]. With the
increasing loading of Co, and RuCo alloy NPs were uniformly dispersed on the
surface of the MIL-96, that caused an increase in the contact area between metal NPs
and AB as well as the number of active sites with the activated transient Metal-H.
Therefore, based on the increasing contact area, the activated transient Metal-H and
the synergistic catalysis effect of alloying Ru with Co in the RuCo@MIL-96 catalyst,
the catalytic activity obviously enhanced with the increase Ru:Co ratio 1:0.5,1:1.
However, more Co loading would result in the excess coverage of Ru active sites [48],
or the reactant bound to the catalyst surface too strongly, that would occupy all
available surface sites and poison the catalyst [49], causing the lower activity for
hydrogen generation from the hydrolysis of AB.
In comparison with the case of the monometallic Ru@MIL-96 and Co@MIL-96
catalyst, all the hydrogen generation rate and productivity were recorded in Fig.8c, it
is obvious that Co@MIL-96 almost have no catalytic activity toward hydrolysis of
AB, and with the same amount of Ru, monometallic catalyst Ru@MIL-96 has much

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inferior catalytic activity than that of Ru1Co1@MIL-96. As can be seen from Fig. 8d,
when the molar ratio of Ru to Co contained the same as that of Ru1Co1@MIL-96, the
catalytic performance of the mixture of monometallic counterparts Ru@MIL-96 and
Co@MIL-96 is obviously inferior to that of the Ru1Co1@MIL-96 catalyst. The result
fully confirms the strong synergistic effects among noble metal Ru and non-noble
metal Co. Moreover, the catalytic activity of Ru1Co1@MIL-96 is superior to that of
pure RuCo NPs, which demonstrates the bi-functional effects between RuCo alloy
NPs and the host of MIL-96 in the process of hydrolysis of AB.
In order to get the activation energy (Ea) of the AB hydrolysis catalyzed by
Ru1Co1@MIL-96, the hydrolytic experiments at different temperatures ranged from
25 to 40 were carried out. Fig. 9a shows that the hydrolytic rates increase with the
increase of reaction temperature, and the values of rate constant k at different
temperature are calculated from the slope of the linear part of each plot. The
Arrhenius plot of lnk vs. 1/T for the catalyst is plotted in Fig. 9b, from which the
apparent activation energy (Ea) is determined to be approximately 36.0 kJ/mol for AB.
Although this value is higher than that reported for several Ru-based catalysts (Table
2), it is lower than most of the reported activation energy values for the same reaction
using many different catalysts even with some noble-metal containing catalysts,
indicating that the as-obtained Ru1Co1@MIL-96 catalyst is very promising candidates
for the hydrolysis of AB.
The durability research is the key point for the practical application of catalysts.
Fig. 10a shows that the Ru1Co1@MIL-96 catalyst still exhibits excellent catalytic
activity after multiple cycles of reaction. Fig. 10b shows the conversion of AB and the
retained percent catalytic activity in the subsequent catalytic runs for the hydrolysis of
AB. The as-synthesized Ru1Co1@MIL-96 catalyst retains about 90.5% of its initial
catalytic activity toward hydrolysis of AB after the second run and the following three
times are 83.6%, 75.7% and 69.7%, respectively. The metal leaching in different uses
were determined to be 6.30%, 9.99%, 12.44%, 14.34% and 16.02%, respectively
(Table 3). The high durability of the catalyst should be attributed to the fact that the
crystalline structure of the catalyst is mostly unchanged and the ultrafine RuCo alloy

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NPs still have been effectively immobilized in the framework of MIL-96 through the
catalytic cycles. The representative TEM image of Ru1Co1@ MIL-96 catalyst after the
fifth durability test is shown in Fig. 4c,d. There is no obvious change in the
morphology of the catalyst and metal NPs still have good load, confirming that
MIL-96 can stabilize RuCo NPs for good durability and confinement effect. Therefore,
the decrease of the catalytic activity might be caused by the increasing viscosity of the
reaction solution and/or the deactivation effect of the increasing metaborate
concentration during the hydrolysis of AB as well as the Ru and Co metal leaching
[18,50]. From Fig. 10c, it can be seen that Ru1Co1@MIL-96 also shows high
reusability during five runs. The life-time of the catalyst in the AB hydrolysis was
determined by calculating the TON, in which the molar ratio of ammonia borane vs
catalyst was controlled more than 50,000. Fig. 10(d) shows the variation of TON
versus time during the hydrolysis of AB, from which we can see that the
Ru1Co1@MIL-96 catalyst has 30400 turnovers in the hydrolysis of AB at 25ꢁ
before deactivated. Based on the observations above, the Ru1Co1@MIL-96 catalyst is
highly stable toward the hydrolytic dehydrogenation of AB.
4. Conclusion
In summary, the RuCo alloy NPs supported on MIL-96 have been designed and
successfully prepared. The obtained Ru1Co1@MIL-96 nanocatalysts exhibit excellent
catalytic activity and durability for hydrolytic dehydrogenation of aqueous AB at
room temperature, owing to its strong bimetallic synergistic effects, uniform
dispersion of NPs on the surface of MIL-96 and bi-functional effects between RuCo
alloy NPs and the host of MIL-96. The activation energy (Ea) and turn over frequency
(TOF) are determined to be 36.0 kJ/mol and 320.7 mol H2 min-1(mol Ru)-1
,
respectively. In addition, this simple synthetic method can be easily extended to facile
preparation other MOFs supported metal NPs. Moreover, more non-noble metal
component in the catalyst not only improves the catalytic activity, but also reduces the
cost which is the key to building a sustainable development of industry in the future.
The superior catalytic performance of the RuCo@MIL-96 makes it an ideal catalyst

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for hydrolysis of AB using as a highly efficient portable hydrogen storage material.
Acknowledgments
This project was supported by the Natural Science Fund for Creative Research
Groups of Hubei Province (2014CFA015) of China. This work was also supported by
the Hubei College Students' Innovation Training Program of China (201410512024,
201510512030).
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Table 1 - Elemental analysis of Ru1Cox@MIL-96 catalysts determined by ICP-AES.
Table 2 - Various catalyst systems tested in the hydrolysis of ammonia borane at room
temperature.
Table 3 - The metal leaching of Ru1Co1@MIL-96 (5 mg) in the subsequent runs of the hydrolysis
of ammonia borane.

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catalysts
Ru1Co0
1:0
Ru1Co0.5
1:0.5
Ru1Co1
1:1
Ru1Co2
1:2
Ru1Co3
1:3
Precursor ratios Ru:Co
Actual ratios Ru:Co
Actual loading of Ru wt%
Actual loading of Co wt%
1:0
1:0.47
1.99
1:0.99
2.07
1:1.93
2.05
1:2.78
1.91
1.88
0
0.55
1.19
2.31
3.09
Table 1

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Ru/AB or
TOFa(mol H2
mol-1 M min-1)
Ea
(kJ mol-1)
Catalyst
Ref.
Metal/AB(mol/mol)
Ru/C
0.004(Ru/AB)
0.02(Metal/AB)
0.004(Ru/AB)
0.0019(Ru/AB)
0.0017(Ru/AB)
0.005(Ru/AB)
0.004(Ru/AB)
0.018(Metal/AB)
0.012(Metal/AB)
0.005(Ru/AB)
0.009(Ru/AB)
0.0083(Metal/AB)
0.003(Metal/AB)
0.018(Metal/AB)
429.5
408.9
339.5
329
34.81
-
[33]
[34]
Pd@Co/graphene
Ru@Ni/graphene
Ru(0)@MWCNTs
RuCo@MIL-96
Ru(0)/TiO2
36.59
33
[35]
[36]
320.7
241
36.0
70
This study
[37]
RuCu/graphene
2 wt.% Pt/C
NiAgPd/C
135
30.39
-
[38]
111
[39]
93.8
83.3
77
38.36
48
[40]
Ru@Al2O3
[41]
Ru/γ-Al2O3
23
[42]
NiCo2O4
50.1
29.9
13.89
17.5
45.9
-
[43]
Cu75Pd25/RGO
Pt black
[44]
[39]
a TOF values were taken directly from the related articles if they were reported. If not, the initial
TOF values were calculated from the corresponding hydrogen generation versus time graphs
presented in the article.
Table 2

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Run
1
Ru leaching (mg)
0.0060
Co leaching (mg)
0.0043
Metal leaching (%)
6.30
9.99
2
0.0102
0.0061
3
0.0131
0.0072
12.44
14.34
16.02
4
0.0155
0.0079
5
0.0176
0.0085
Table 3

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Figure captions:
Fig. 1. View of the corrugated chains in MIL-96.
Fig. 2. Low-angle (A), wide-angle (B) and the magnified Ru (C) powder XRD patterns of samples:
(a) MIL-96; (b) Ru@MIL-96; (c) Co@MIL-96; (d) Ru1Co1@MIL-96; (e) Ru1Co1@MIL-96 after
the fifth run.
Fig. 3. XPS spectra of Ru1Co1@MIL-96: (a) as-prepared; (b) Ru 3p; (c) Co 2p.
Fig. 4. TEM images of (a-b) as-prepared Ru1Co1@MIL-96; (c-d) Ru1Co1@MIL-96 after the fifth
run; particle size distribution of (e) as-prepared Ru1Co1@MIL-96; (f) EDX spectrum of Ru1Co1@
MIL-96; particle size distribution of (g) Ru@MIL-96; (h) Co@MIL-96; (i) Ru1Co1@MIL-96 after
the fifth run.
Fig. 5. N2 sorption isotherms of (a) MIL-96 and (b) as-prepared Ru1Co1@MIL-96; (c) pore size
distribution of MIL-96 and Ru1Co1@MIL-96.
Fig. 6. FT-IR of (a) MIL-96; (b) initial Ru1Co1@MIL-96; (c) Ru1Co1@MIL-96 after the fifth run.
Fig. 7. Hydrogen generation from the hydrolysis of AB catalyzed by MIL-96, Ru@MIL-96,
Co@MIL-96, Ru1Co1@MIL-96 and RuCo NPs (5mg catalysts, 18.5mg AB at 25 ).
Fig. 8. (a) Hydrogen generation rate from the hydrolysis of AB catalyzed by different Ru, Co
based catalysts; (b) the plot of hydrogen generation rate versus Co molar ratio for the hydrolytic
dehydrogenation of AB; (c) hydrogen generation rate from the hydrolysis of AB catalyzed by
Ru@MIL-96, Co@MIL-96 and Ru1Co1@MIL-96; (d) hydrogen generation rate from the
hydrolysis of AB in the presence of Ru1Co1@MIL-96, RuCo NPs or the physical mixture of
Ru@MIL-96+Co@ MIL-96 (5mg catalysts, 18.5mg AB at 25 ).
Fig. 9. (a) Temperature effects on hydrogen generation rate catalyzed by Ru1Co1@MIL-96 from
AB hydrolysis (5mg catalysts, 18.5mg AB) in the range of 25-40 and (b) Arrhenius plot
obtained from temperature effects.
Fig. 10. (a) Durability of the Ru1Co1@MIL-96 catalyst from 1st to 5th cycles; (b) percentage of
initial catalytic activity of Ru1Co1@MIL-96 in successive runs for the hydrolysis of AB; (c) The
plot of mol H2 per mol AB versus time for hydrolysis of AB in the presence of Ru1Co1@MIL-96
during a five-cycle reusability test; (5mg catalysts, 18.5mg AB at 25℃) (d) The turnover number
(TON) versus time plot for Ru1Co1@MIL-96.

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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

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