- Email: [email protected]
CuNi NPs supported on MIL-101 as highly active catalysts for the hydrolysis of ammonia borane
Accepted Manuscript Title: CuNi NPs supported on MIL-101 as highly active catalysts for the hydrolysis of ammonia borane Authors: Doudou Gao, Yuhong Zhang, Liqun Zhou, Kunzhou Yang PII: DOI: Reference:
S0169-4332(17)32525-4 http://dx.doi.org/10.1016/j.apsusc.2017.08.167 APSUSC 37004
To appear in:
APSUSC
Received date: Revised date: Accepted date:
25-7-2017 16-8-2017 24-8-2017
Please cite this article as: Doudou Gao, Yuhong Zhang, Liqun Zhou, Kunzhou Yang, CuNi NPs supported on MIL-101 as highly active catalysts for the hydrolysis of ammonia borane, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.08.167 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
CuNi NPs supported on MIL-101 as highly active catalysts for the hydrolysis of ammonia borane Doudou Gao, Yuhong Zhang, Liqun Zhou, Kunzhou Yang Hubei 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, China
Graphical abstract
The Scheme of the AB hydrolysis catalyzed by CuxNi1@MIL-101.
Corresponding author. E-mail address: [email protected] (L. Zhou).
Highlights 1. Cu2Ni1@MIL-101 shows good catalytic activity and durability for hydrolysis of AB. 2. There are strong synergistic effects between metals Cu and Ni nanoparticles. 3. The bi-functional effects between CuNi nanoparticles and MIL-101 are significant.
ABSTRACT: The catalysts containing Cu, Ni bi-metallic nanoparticles were successfully synthesized by in-situ reduction of Cu2+ and Ni2+ salts into the highly porous and hydrothermally stable metal-organic framework MIL-101 via a simple liquid impregnation method. When the total amount of loading metal is 3×10-4mol, Cu2Ni1@MIL-101
catalyst
shows
higher
catalytic
activity
comparing
to
CuxNiy@MIL-101 with different molar ratio of Cu and Ni (x, y= 0, 0.5, 1.5, 2, 2.5 ,3). Cu2Ni1@MIL-101 catalyst has the highest catalytic activity comparing to mono-metallic Cu and Ni counterparts and pure bi-metallic CuNi nanoparticles in hydrolytic dehydrogeneration of ammonia borane (AB) at room temperature. Additionally, in the hydrolysis reaction, the Cu2Ni1@MIL- 101 catalyst possesses excellent catalytic performances, which exhibit highly catalytic activity with turn over frequency (TOF) value of 20.9 mol H2 min-1 Cu mol-1 and a very low activation energy value of 32.2 kJ mol-1. The excellent catalytic activity has been successfully achieved thanks to the strong bi-metallic synergistic effects, uniform distribution of nanoparticles and the bi-functional effects between CuNi nanoparticles and the host of MIL-101. Moreover, the catalyst also displays satisfied durable stability after five cycles for the hydrolytically releasing H2 from AB. The non-noble metal catalysts have broad prospects for commercial applications in the field of hydrogen-stored materials due to the low prices and excellent catalytic activity.
Keywords: CuNi@MIL-101; Heterogeneous catalysis; Ammonia-borane; Releasing hydrogen
1. Introduction Hydrogen is an environmentally friendly fuel and regarded as a new generation of the ideal alternative energy sources [1]. Currently, various hydrogen storage approaches have been investigated, including metal hydrides [2], sorbent materials [3], and coordination of hydride systems [4]. In these materials, ammonia borane (NH3BH3, denoted as AB) is nontoxic, stable, environmentally benign, and is considered to be an attractive solid hydrogen storage materials due to its high hydrogen mass capacity, high solubility and demonstrated stability in neutral aqueous solutions [5]. The catalytic hydrolysis reaction can be briefly expressed as follows: NH3 BH 3 2H 2O Cata . NH 4 BO2 3H 2
So far, the optimal compromise between costs, efficiency and recyclability still remains a considerable challenge for the practical use of AB as a potential hydrogen storage material. Up to now, a series of noble metal NPs@MOFs catalysts were tested for hydrolytic dehydrogenation of AB [6]. Moreover, it has very large windows and surface areas with Slangmuir = 3750 m2 g-1, which easily allow the introduction of new species into the cages. These properties, together with high adsorption capacities, make MIL-101 become an attractive candidate for the host of the NPs@MOFs catalysts [7]. Recently, a series of metal NPs@MIL-101 catalysts were tested for the hydrolytic dehydrogenation of AB, such as Pd@MIL-101[8], Ru@MIL-101[9], CuPd/MIL-101, PdNi@MIL-101[10], and RuCuCo@MIL-101 and so on. However, the widespread utilization of noble metal-based catalysts was hindered by their high price and limited abundance, and the research on the catalysts of non-noble metal NPs@MOFs were rarely reported. Especially, the non-noble bi-metallic synergistic effects can improve the catalytic activity. Therefore, the synthesis of catalysts from non-noble bi-metallic nanoparticles supported on MOFs is our purpose. In this work, we report the generation of hydrogen from hydrolysis of AB over non-noble bi-metallic CuNi nanoparticles immobilized in MIL-101 as efficient catalysts at room temperature, as shown in scheme 1. The structure, morphology, size, composition and specific area of the catalysts with different CuNi nanoparticles loading were discussed in detail, the catalytic activity and
influence factors as well as stability were investigated by comparing the catalysts for hydrolysis of AB. In addition, the activation energy (Ea) for the hydrolysis reaction has been measured by different temperature experiment.
Scheme 1. Cu (NO3)2·3H2O and Ni (NO3)2·6H2O were in-situ reduced by NaBH4 and the hydrolytic dehydrogeneration of ammonia borane (AB) at room temperature catalyzed by CuNi@MIL-101.
2. Experimental 2.1. Synthesis of MIL-101 MIL-101 was synthesized according to hydrothermal method [11, 12]. Terephthalic acid (2007.0 mg, 5.0 mmol), Cr(NO3)3·9H2O (823.0 mg, 5.0 mmol), aqueous HF (0.12 mL, 40 wt%) and de-ionized water (24 mL) were placed in a 40 mL Teflon-liner autoclave and heated at 200 °C for 8 h. After natural cooling, the resulting green powder was doubly filtered off using a filter cloth with pore sizes between 40 and 100 um to remove the unreacted terephthalic acid crystals and then purified by a magnetic stirring at 25 °C for 24 h in ethanol and solvothermal treatment in NH4F solution (30 mM) at 60 °C for 10 h. Finally, the purified MIL-101 was dried overnight
at 80 °C in a vacuum drying oven. 2.2. Synthesis of CuNi@MIL-101 and other catalysts 200 mg of MIL-101 was added to a flask containing 30 mL of de-ionized water. Ultrasonication was required to get a uniform dispersion. Next, Ni (NO3)2·6H2O and Cu (NO3)2·3H2O were 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. CuNi@MIL-101 catalyst was collected through filtration and dried in the vacuum oven at 80°C for 12 h. During the synthesis of mono-metal Cu@MIL-101, Ni@MIL-101 and pure CuNi NPs catalysts, the amounts of Cu and Ni were consistent with that of Cu2Ni1@MIL-101. And CuNi NPs was prepared as follows: Cu (NO3)2·3H2O (0.2 M, 1 ml) and Ni (NO3)2·6H2O (0.1 M, 1 ml) 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 NaBH4 (10 mL, 0.013 mol L-1) 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°C overnight in a vacuum oven for further use. The designed condition for the synthesis of different metal loading catalysts is shown in Table 1.
Table 1.Designed conditions for the synthesis of different metal loading catalysts Cu (10-4 mol) Ni (10-4 mol)
2.5 0.5
2 1
1.5 1.5
1 2
0.5 2.5
3 0
0 3
2 0
0 1
2.3. Catalytic performance test The experiment was conducted in a two-necked round bottom flask equipped with a gas burette and a pressure-equalization funnel to introduce 20 mL de-ionized water. In all the experiments, a mixture of 50.0 mg CuxNiy@MIL-101 and 18.5 mg AB were kept in the flask. When the de-ionized water was added to the above mixture, the reaction of AB hydrolysis rapidly started, and the volume of the generated hydrogen gas was recorded by the displacement level of water in the burette at a certain time interval. A
water bath was used to control the temperature of the reaction solution. In order to get the activation energy (Ea), the reaction was carried out at different temperatures (25, 30, 35 and 40°C) with CuNi@MIL-101 catalyst. 2.4. Stability test The durability of Cu2Ni1@MIL-101 catalyst in the hydrolysis of AB was evaluated by performing the recyclability tests at room temperature. In a typical test, 10 mL of Cu2Ni1@MIL-101 (50 mg) suspension solution and 18.5 mg AB (10 mL, 0.6 mmol) were added into the reaction flask at 25°C, and the evolution of gas was measured as described above. After the hydrolysis reaction was completed, another equivalent of AB (18.5 mg) 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°C. In a typical reusability test, 18.5 mg AB (10 mL, 0.6 mmol) was added to 10 mL of Cu2Ni1@MIL-101 (50 mg) suspension solution, the evolution of gas was monitored as
described
above.
After
the
hydrolysis
reaction
was
completed,
the
Cu2Ni1@MIL-101 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 °C. 2.5. Characterization X-ray powder diffractions (XRD) of products were examined on a diffractometer (Bruker D8 Advance, Germany) with Cu Ka (40 kV, 40 mA) radiation (λ=0.1540 nm) to identify the phase purity and crystallinity. The surface area measurements were performed with N2 adsorption-desorption isotherms were obtained at 77 K using a Micromeritics ASAP 2020 instrument. The metal loading of the prepared catalyst was determined by an IRIS Intrepid IIXSP inductively coupling plasma-atomic emission spectrometer (ICP-AES). The electronic states were investigated by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi). The morphology 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. The Fourier transform infrared (FTIR) spectra of the products were recorded in KBr pellets on a spectrometer (FTIR- Spectrum one, USA) in transmission mode ranging from 500 to 4000 cm–1 under ambient conditions.
3. Results and discussion The XRD patterns of the as-prepared MIL-101, Cu@MIL-101, Ni@MIL-101, Cu2Ni1@ MIL-101 and Cu2Ni1@MIL-101 after the fifth run are illustrated in Fig. 1. From the wide-angle XRD in Fig. 1a, it can be seen that the XRD pattern of MIL-101 is in good agreement with that reported by Férey et al [11], confirming that MIL-101 was successfully synthesized. The Cu@MIL-101, Ni@MIL-101 and Cu2Ni1@MIL-101 and Cu2Ni1@ MIL-101 after the fifth run are matched with those of the host MIL-101, indicating that the integrity of the MIL-101 framework is unchanged during the catalyst preparation and catalytic process, and MIL-101 can provide a good 3D framework for CuNi NPs. In addition, as shown in Fig. 1b, the intensity of the diffraction peaks of the host MIL-101 decreases substantially with the metal nanoparticles loading, this result can be explained by the changes in the charge distribution and electrostatic fields as a result of the existence of Cu Ni nanoparticles on the surface or in the cages and interaction of their electrophilic surface with framework atoms [13]. Additionally, it can be found that there was no significant diffraction peak about metal Cu and Ni species in Fig 1. This may be attributed to low metal loading and the ultrafine nanoparticles into the pores of MIL-101[14, 15].
Fig. 1. (a) Wide-angle powder X-ray diffraction patterns (b) low-angle powder X-ray diffraction
patterns
of
samples
MIL-101,
Cu@MIL-101,
Ni@MIL-101,
Cu2Ni1@MIL-101 and Cu2Ni1@MIL-101 after the fifth run
The X-ray photoelectron spectroscopy (XPS) was employed to demonstrate metal electronic states of the Cu2Ni1@MIL-101 catalyst. Fig. 2a shows the characteristics peaks of C 1s, O 1s, Ni 2p and Cu 2p. The Cu 2p signal is shown in Fig. 2b. Peaks with binding energies of 935.1 eV and 954.6 eV respectively [16]. The shake-up features around 943.2 and 962.6 eV for the Cu 2p3/2 and Cu 2p1/2, respectively, are compelling evidence and diagnostic of an open 3d9 shell, corresponding to Cu (II) [17]. The peak
fit of Cu 2p3/2 reveals three peaks at 935.3, 934.9 and 932.8 eV, which are assigned to Cu(0), Cu(II) and Cu(I) states, respectively, and the shake-up satellite peaks observed at 942.4 and 944.2 eV are indicative of Cu(II) [18,19]. Fig. 2c shows the two peaks centered at 856.5 and 873.6 eV, which can be attributed to Ni (0) 2p3/2 and Ni (0) 2p1/2, respectively. Peaks with binding energies of 862.4 and 879.0 eV, can be indexed to Ni (II) 2p3/2 and Ni (II) 2p1/2, respectively, such as NiO and Ni (OH) 2 [20]. These results indicate that Ni (0) and Cu (0) can be oxidised during the XPS sampling [21].
Fig.2. XPS spectra of Cu2Ni1@MIL-101: (a) full spectrum; (b) Cu 2p; (c) Ni 2p The morphologies and loading effect of Cu, Ni and CuNi nanoparticles immobilized in MIL-101 were further characterized by TEM. The TEM images of MIL-101, Cu2Ni1@MIL-101, Cu@MIL-101 and Ni@MIL-101 catalysts were shown in Fig. 3a, b, c, d, respectively. As can be seen in Fig. 3a, before loading the metal NPs, the surface of MIL-101 is smooth without any aggregates. Fig. 3b shows that the CuNi nanoparticles are uniformly loaded on the surface of MIL-101 and no significant aggregate was formed, which is the basis for high performance of the catalyst, the average particle size in the Cu2Ni1@MIL-101 catalyst was about 1.75 nm, as shown in Fig. 3f. Comparing to Fig 3b, in Fig. 3c, it can be seen that most of mono-metallic Cu nanoparticles were agglomerated on the surface of MIL-101, and Fig. 3d shows that the Ni nanoparticles were uniformly loaded on the surface of MIL-101 but with less loading, which reduced effective contact area with reactants and interpreted the low catalytic efficiency of mono-metallic catalysts [11, 22]. The mean particle sizes of the metal nanoparticles in Cu@MIL-101 and Ni@MIL-101 were about 7.0 nm and 3.5 nm, respectively, as shown in Fig. 3g, h. In order to study the catalyst recyclability, the TEM of Cu2Ni1@MIL-101 after five cycles were further characterized, see Fig.3e, which shows that after five cycles, there are still a great number of metal nanoparticles loading on MIL-101 surface. Moreover, the MIL-101 framework is maintained well which is the guarantee of excellent durability.
Fig. 3. TEM images of (a) MIL-101; (b) Cu2Ni1@MIL-101(c) Cu@MIL-101; (d) Ni@MIL-101(e) Cu2Ni1@MIL-101after fifth run; particle size distributions of (f) Cu2Ni1@MIL-101; (g) Cu@MIL-101; (h) Ni@MIL-101.
Fig. 4. N2 sorption isotherms of (a) MIL-101, (b) Cu2Ni1@MIL-101 Fig.4 shows the N2 sorption isotherms of (a) MIL-101 and (b) Cu2Ni1@MIL-101. The Brauner-Emmett-Teller (BET) specific surface area of the MIL-101 was 3170 m2 g-1, which is close to the values reported in the literature [11]. However, after the loading of CuNi nanoparticles, the surface area of MIL-101 dropped significantly from 3170m²g-1 to 1317 m²g-1. This phenomenon fully confirmed that the partial small size nanoparticles embedded in the pore of MIL-101, owing to the pore diameter of MIL-101 is about 2.3 nm [11, 12] and the size of CuNi NPs are in the range of 0.5-3.0 nm. After loading metal NPs, the small size metal nanoparticles would occupy or block the cages of MIL-101 and the big size nanoparticles are mainly deposited on the
surface of MIL-101 and directly lead to a decrease in the specific surface area, which has been proved by previous work [9, 10, 23]. In order to further illustrate the presence of Cu and Ni elements in the catalyst, EDX characterization was also performed, as shown in Fig. 5. The results show that the chemical composition of the product is element Cu, Ni, Cr, C, O. In addition, the actual loading of the metal Cu and Ni in the catalyst Cu2Ni1@MIL-101 was determined to be 3.4 wt% and 1.4 wt% by ICP-AES, which can explain the existence of Cu and Ni, as shown in Table 2. The results show that the actual ratios of Cu: Ni is almost similar to the precursor ratios in the CuxNiy@MIL-101 catalysts.
Fig. 5. EDX spectrum of Cu2Ni1@MIL-101.
Table 2. Elemental analysis of CuXNiy@MIL-101 catalysts determined by ICP-AES. Catalysts
Cu2.5Ni0.5
Cu2Ni1
Cu1.5Ni1.
Cu1Ni2
Cu0.5Ni2.5
5
Precursor ratios Cu:Ni Actual ratios Cu:Ni Actual loading of Cu wt% Actual loading of Ni wt%
5:1 4.3 : 1 2.2 % 0.5 %
2:1 2.3 : 1 3.4 % 1.4 %
1:1 1:1 2.9 % 2.7 %
1:2 1 : 2.1 0.7 % 1.4 %
1:5 1 : 4.5 0.6 % 2.7 %
Fig. 6 shows the hydrogen generation volumes as the function of reaction time obtained by the hydrolysis of AB in the CuxNiy@MIL-101 catalysts. The results are recorded in Fig. 6a. It is evident that, by changing the molar ratios 2.5:0.5, 2:1, 1.5:1.5, 1:2 to 0.5:2.5 of Cu and Ni, the bi-metallic nanoparticles show different catalytic
activities. Research shows that when the total amount of loading metal was 3×10-4mol, the catalyst Cu2Ni1@MIL-101 and Cu1.5Ni1.5@MIL-101 show the similar catalytic activity at the beginning and the end of the reaction. But in the middle stage of the reaction, Cu2Ni1@MIL-101 exhibited higher catalytic activity than Cu1.5Ni1.5@MIL -101, with the turn over frequency (TOF) value of 20.9 mol H2 min-1 Cu mol-1. Comparing to the other catalyst used in the same reaction, it can be seen that as-synthesized Cu2Ni1@MIL-101 catalyst has a higher TOF value and satisfied catalytic activity. In order to further study the synergistic effects among non-noble metal Cu and Ni, we measured the catalytic activity of mono-metallic counterparts catalysts, the results were shown in Fig. 6b, when the same amount of total metal, in comparison with the case of the mono-metallic Cu3@MIL-101 or Ni3@MIL-101 catalyst, a high hydrogen generation rate was observed for the corresponding Cu2Ni1@MIL-101. Moreover, the catalytic performance of the mixture of mono-metallic counterparts Cu2@MIL-101 + Ni1@MIL-101 was obviously inferior to that of the Cu2Ni1@MIL- 101. These results fully suggest that the strong synergistic interaction between Cu and Ni plays an important role in the hydrolysis of NH3BH3. Furthermore, similar synergetic features in bi-metallic NPs were found in our previous work and literatures [23, 24, 25, 26]. For alloy NPs, synergetic effects may be mainly ascribed to electronic effects, which are important when the metal atoms have significant differences in electronegativity [26], and geometric effects. In the present case, the observed synergetic effects in catalysis might be mainly due to the geometric effects, not the electronic effects, because the electronegativities of Cu (1.90) and Ni (1.91) are very close. In the geometric effects, bi-metallic synergistic effects may be related to the metal crystal structure of Cu and Ni NPs, Cu and Ni both are A1-type crystal structure, When forming CuNi alloy NPs, Ni replace some Cu in Cu lattice and keep the crystal system of Cu sustaining the A1-type crystal structure, which makes Cu and Ni exhibit good synergistic effects.
Fig.6. Hydrogen generation from the hydrolysis of AB (a) catalyzed by Cu2.5Ni0.5@MIL-101, Cu2Ni1@MIL-101, Cu1.5Ni1.5@MIL-101, Cu1Ni2@MIL-101 and Cu0.5Ni2.5 @MIL-101; (b) catalyzed by Cu2Ni1@MIL-101, Cu@MIL-101, Ni@MIL-101
and
Cu2@MIL-101
+
Cu2Ni1@MIL-101, MIL-101 and CuNi NPs.
Ni1@MIL-101;
(c)
catalyzed
by
In addition, we conducted a thorough study of the bi-functional effects between CuNi nanoparticles and the host of MIL-101, as shown in Fig. 6c, the catalytic activity of Cu2Ni1@MIL-101 is superior to that of pure Cu2Ni1 nanoparticles. Furthermore, it is obvious that the pure MIL-101 almost have no catalytic activity toward hydrolysis of AB, and the poor catalytic activity has been proved by previous work [24].Those results fully demonstrate that the bi-functional effects between CuNi alloy NPs and MIL-101 significantly enhanced the reaction activity of Cu2Ni1@ MIL-101 catalyst, in which MIL-101 can provide a good 3D framework for CuNi NPs in spite of it does not involved in the hydrolysis reaction of AB. From Fig.6, it can be found that the reaction rate increased at first and then decreased when reached a certain time. This phenomenon may relevant to catalytic mechanism of the CuNi@MIL-101 catalysts for the hydrolysis reaction of AB. For the present heterogeneous catalytic reaction, there seems no doubt that the catalytic reaction takes place on the surface area of the metal catalyst [27]. The mechanism involves two steps. Firstly, AB interacts with the surface of CuNi nanoparticles 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 [28, 29]. In order to get the activation energy (Ea) of the AB hydrolysis catalyzed by Cu2Ni1@MIL-101, the hydrolytic experiments at different temperatures ranged from 25 to 40 °C were carried out. Fig. 7a shows that the hydrolytic rates increase with the increase of reaction temperature, and the values of rate constant k at different temperatures 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. 7b, from which the apparent activation energy (Ea) is determined to be approximately 32.2 kJ mol-1 for AB. By comparing the activation energy and turn over frequency of non-noble metal catalyst Cu2Ni1@MIL-101 with that of noble metal catalysts in Table 3, we can see that the Cu2Ni1@MIL-101 shows the good catalytic activity and the low activation energy. The results indicate that the as-synthesized Cu2Ni1@MIL-101 catalyst is a very promising candidate for the hydrolysis of AB.
Fig. 7. (a) Hydrogen evolution rate of Cu2Ni1@MIL-101 for the hydrolysis of AB at different temperatures; and (b) Arrhenius curves. Additionally, the as-synthesized Cu2Ni1@MIL-101 was tested in terms of durability in five cyclic uses, as shown in Fig. 8. The catalyst still exhibits high catalytic activity after five cycles, retaining 75% of its initial catalytic activity, see Table 3, the Cu2Ni1@MIL-101 has a good durability by comparing with that of various noble metal catalysts, which makes Cu2Ni1@MIL-101 very conducive in the practical application. The high durability should be attributed to the crystalline structure of the catalyst is mostly unchanged and the ultrafine CuNi NPs still effectively immobilized in MIL-101 through the catalytic cycles. In Fig. 9, the intensity and position of FT-IR peaks of MIL-101, initial Cu2Ni1@MIL-101 and Cu2Ni1@MIL-101 after the fifth run
show no obvious change. This phenomenon indicates that the crystalline structure of the host of MIL-101 is very stable, that provides favorable conditions for CuNi NPs effectively immobilized in the framework of MIL-101 after the catalytic cycles [40].
Fig. 8. (a) The time course plots for durability of Cu2Ni1@MIL-101; (b) Percentage of initial catalytic activity of Cu2Ni1@MIL-101 in successive runs after the reuse for the hydrolysis of AB.
Table 3.Various catalyst systems tested in the hydrolysis of AB at room temperature. Catalyst Pt black
Ea (kJ
TOF (mol H2
mol-1
)
min-1 M mol-1 )
Durability
Ref.
-
13.89
-
[30]
Pd@Co/graphene Ag/C/Ni Cu75Pd25/RGO
38.9 45.9
37.5 5.32 29.9
59% (5 runs) 79% (4 runs) -
[31] [32] [33]
CoPd/C Pd10Ni6@MIL-101 2.1 wt% Pd@RGO
27.5 31.7 40
22.7 83.1 26.3
85% (5 runs) 65% (3 runs)
[34] [23] [35]
Pd-PVP-TiO2
55.9
3.1
89% (5 runs)
[36]
rGO-Ni30Pd70 Co-Ni-P/Pd-TiO2 PVP-stabilized Pd Pd/zeolite Cu2Ni1@MIL-101
45 54.7 35 56 32.2
28.7 0.3 22.3 6.25 20.9
63% (3 runs) 69% (5 runs) 65% (5runs) 75% (5 runs)
[37] [38] [39] [21] This study
Fig. 9 FT-IR of (a) MIL-101; (b) initial Cu2Ni1@MIL-101; (c) Cu2Ni1@MIL-101 after the fifth run.
The reduction of catalytic activity may be due to the increasing viscosity of the solution or the deactivation effect of the increasing metaborate concentration during the hydrolysis [22, 41]. Besides, the catalytic activity decrease may be related to the metal
leaching. In Table 4, metal leaching in the five-times used were determined to be 7.7%, 9.7%, 12.1%, 14.8% and 17.6%, respectively. Table 4. The metal leaching of Cu2Ni1@MIL-101 (50mg) in the subsequent runs of the hydrolysis of ammonia borane. Run
Cu leaching (mg)
Ni leaching (mg)
Metal leaching(%)
1 2
0.165 0.207
0.018 0.025
7.7 9.7
3
0.256
0.032
12.1
4 5
0.315 0.375
0.037 0.045
14.8 17.6
4. Conclusions In summary, MIL-101 supported CuNi nanoparticles catalysts have been successfully synthesized, which exhibit highly catalytic activity with turn over frequency (TOF) value of 20.9 mol H2 min-1 Cu mol-1 and the good durability, maintaining 75% after fifth runs. In addition, the activation energy (Ea) was determined to be 32.2 kJ mol-1. The excellent catalytic performances of catalytic hydrolysis of AB were owing to the strong bi-metallic synergistic effects, uniform distribution of nanoparticles as well as bi-functional effects between CuNi NPs and the host of MIL-101. In addition, as the price of noble metals reached their historical height, non noble metal component in catalyst not only improves the catalytic activity, but also reduces the cost which is a key for industry for a sustainable future. Therefore, easy preparation and the high catalytic performances reveal that the CuNi@MIL-101 catalyst is a promising candidate to be employed in developing AB as a highly efficient, portable hydrogen storage system.
Acknowledgments This project was supported by the Natural Science Fund for Creative Research Groups of Hubei Province (2014CFA015), Hubei Province Education Office Key
Laboratory (2016-KL-007) of China. This work was also supported by the Hubei College Students' Innovation Training Program of China (201410512024, 201510 512030).
References [1] S. Ma, J. Xie, J. Wen, K. He, X. Li, W. Liu, X. Zhang, Constructing 2D layered hybrid CdS nanosheets/MoS2 heterojunctions for enhanced visible-light photocatalytic H2 generation. Appl. Surf. Sci. 391 (2017) 580-591. [2] Y. Wan, D. Zhao, On the controllable soft-templating approach to mesoporous silicates, Chem. Rev. 107 (2007) 2821-2860. [3] KSW. Sing, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984), Plast. Reconstr. Surg. 57 (1985) 603-619. [4] M. E. Bluhm, M. G. Bradley, R. Butterick, U. Kusari, L. G. Sneddon, Amineborane-based chemical hydrogen storage: enhanced ammonia borane dehydrogenation in ionic liquids, J. Am. Chem. Soc. 128 (2006) 7748-7749. [5] B. Peng, J. Chen, Ammonia borane as an efficient and lightweight hydrogen storage medium, Energ. Environ. Sci. 1 (2008) 479-483. [6] J. Shen, L. Yang, K. Hu, W. Luo, G. Cheng, Rh nanoparticles supported on graphene as efficient catalyst for hydrolytic dehydrogenation of amine boranes for chemical hydrogen storage，Int. J. Hydrogen Energ. 40(2015) 1062-1070. [7] R. Fang, H. Liu, R. Luque, Y. Li, Efficient and selective hydrogenation of biomass-derived furfural to cyclopentanone using Ru catalysts, Green Chem. 17 (2015) 4183-4188. [8] Y. Zhao, M. Liu, B. Fan, Y. Chen, W. Lv, N. Lu, R. Li, Pd nanoparticles supported on ZIF-8 as an efficient heterogeneous catalyst for the selective hydrogenation of cinnamaldehyde, Catal. Commun. 57 (2014) 119-123. [9] T. Liu, Q. Wang, B. Yan, M. Zhao, W. Li, H. Bie, Ru nanoparticles supported on MIL-101 by double solvents method as high-performance catalysts for catalytic hydrolysis of ammonia borane, J. Nanomater. 2015 (2015) 679526 (1-5). [10] A. Dhakshinamoorthy, H. Garcia, Catalysis by metal nanoparticles embedded on metal–organic frameworks, Chem. Soc. Rev. 41 (2012) 5262-5284. [11] G. Ferey, C. Mellotdraznieks, C. Serre, F. Millange, J. Dutour, S. Surble, I. Margiolaki, A chromium terephthalate-based solid with unusually large pore volumes and surface area, Science 309 (2005) 2040-2042. [12] D. Hong, H. Wang, YK, C. Serre, G. Ferey, J. Chang, Porous chromium terephthalate MIL‐ 101 with coordinatively unsaturated sites: surface functionalization, encapsulation, sorption and catalysis, Adv. Funct. Mater. 19 (2009) 1537-1552.
[13] T. Loiseau, L. Lecroq, C. Volkringer, J. Marrot, G. Ferey, M. Haouas, F. Taulelle, S. Bourrelly, P. Llewellyn, M. Latroche, MIL-96, a porous aluminum trimesate 3D structure constructed from a hexagonal network of 18-membered rings and mu3-oxo-centered trinuclear units, J. Am. Chem. Soc. 128 (2006) 10223-10230. [14] M. Yurderi, A. Bulut, M. Zahmakiran, M. Gulcan, S. Özkar, Ruthenium(0) nanoparticles stabilized by metal-organic framework (ZIF-8): Highly efficient catalyst for the dehydrogenation of dimethylamine-borane and transfer hydrogenation of unsaturated hydrocarbons using dimethylamine-borane as hydrogen source, Appl. Catal. B: Environ. 160 (2014) 534-541. [15] M. S. El-shall, V. Abdelsayed, A.E.R.S. Khder, H.M.A. Hassan, H.M. El-kaderi, T. E. Reich, Metallic and bimetallic nanocatalysts incorporated into highly porous coordination polymer MIL-101, J. Mater. Chem. 19 (2009) 7625-7631. [16] C. C. Chusuei, M. A.Brookshier, D. W. Goodman, Correlation of relative X-ray photoelectron spectroscopy shake-up intensity with CuO particle size, Langmuir 15 (1999) 2806-2808. [17] D. Bazin, I. Kovacs, JA. Lynch, L. Guczi, Ru-Co/NaY bimetallic catalysts: in situ EXAFS study at Co K- and Ru K-absorption edges, Appl. Catal. A: Gen. 242 (2003) 179-186. [18] G. Wang, B. Huang, Z. Lou, Z. Wang, X. Qin, X. Zhang, Y. Dai, Valence state heterojunction Mn3O4/MnCO3: Photo and thermal synergistic catalyst, Appl. Catal. B: Environ. 180 (2016) 6-12. [19] W. Wang, G. Wang, X. Wang, Y. Zhan, Y. Liu, C. Zheng, Synthesis and characterization of Cu2O nanowires by a novel reduction route, Adv. Mater. 14 (2002) 67-69. [20] M. Zahmakıran, T. Ayvalı, S. Akbayrak, S. Çalışkan, D. Çelik, S. Özkar, Zeolite framework stabilized nickel (0) nanoparticles: active and long-lived catalyst for hydrogen generation from the hydrolysis of ammonia-borane and sodium borohydride, Catal. Today 170 (2011) 76-84. [21] K. B. Hong, C. W. Lee, E. K. Yum, Synthesis of 2-substituted indoles by palladium-catalyzed heteroannulation with Pd-NaY zeolite catalysts. Tetrahedron Lett. 35(2004) 693-697. [22] N. Cao, T. Liu, J. Su, X. Wu, W. Luo, G. Cheng, Ruthenium supported on MIL-101 as an efficient catalyst for hydrogen generation from hydrolysis of amine boranes, New. J. Chem. 38 (2014) 4032-4035. [23] L. Zhang, L. Zhou, K. Yang, D. Gao, C. Huang, Y. Chen, F. Zhang, X. Xiong, L. Li, Q. Xia, Pd Ni nanoparticles supported on MIL-101 as high-performance catalysts for hydrogen generation from ammonia borane, J. Alloy. Compd. 677 (2016) 87-95. [24] Z. H. Lu, J. P. Li, G. Feng, Q. L. Yao, F. Zhang, R. Y. Zhou, D. J. Tao, X. S. Chen, Z. Q. Yu, Synergistic catalysis of MCM-41 immobilized Cu-Ni nanoparticles in hydrolytic dehydrogeneration of ammonia borane, Int. J. Hydrogen Energy 39 (2014) 13389-13395.
[25] 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, J. Alloy. Compd. 694 (2017) 662-671. [26] J. Li, L. Zhu, Q. Xu, Non-noble bimetallic CuCo nanoparticles encapsulated in the pores of metal–organic frameworks: synergetic catalysis in the hydrolysis of ammonia borane for hydrogen generation, Catal Sci Technol. 5(2015) 525-530. [27] D. Ke, Y. Li, J. Wang, L. Zhang, J. Wang, X. Zhao, S. Yang, S. Han, Facile fabrication of poly(amidoamine) (PAMAM) dendrimers-encapsulated Ag–Co bimetallic nanoparticles for highly efficient dehydrogenation of ammonia Borane, Int. J. Hydrogen. Energ. 41 (2016) 2564-2574. [28] Q. Xu, M. Chandra, Catalytic activities of non-noble metals for hydrogen generation from aqueous ammonia–borane at room temperature, J. Power Sources 163 (2006) 364-370. [29] G. Guella, C. Zanchetta, B.R. Patton, A. Miotello, New insights on the mechanism of palladium-catalyzed hydrolysis of sodium borohydride from 11B NMR measurements, J. Phys. Chem. B 110 (2006) 17024-17033. [30] H. Liang, G. Chen, S. Desinan, R. Rosei, F. Rosei, D. Ma, In situ facile synthesis of ruthenium nanocluster catalyst supported on carbon black for hydrogen generation from the hydrolysis of ammonia-borane, Int. J. Hydrogen Energ. 37 (2012) 17921-17927. [31] N. Cao, J. Su, W. Luo, G. Cheng, Hydrolytic dehydrogenation of ammonia borane and methylamine borane catalyzed by graphene supported Ru @Ni core–shellnanoparticles, Int. J. Hydrogen Energ. 39 (2014) 426-435. [32] M. Wen, B. Sun, B. Zhou, Q. Wu, J. Peng, Controllable assembly of Ag/C/Ni magnetic nanocables and its low activation energy dehydrogenation catalysis. J. Mater. Chem. 22 (2012) 11988-11993. [33] K. Güngörmez, Ö. Metin, Composition-controlled catalysis of reduced graphene oxide supported CuPd alloy nanoparticles in the hydrolytic dehydrogenation of ammonia borane. Appl. Catal. A-Gen. 494 (2015) 22-28. [34] L. Hu, B. Zheng, Z. Lai, K-W. Huang, Room temperature hydrogen generation from hydrolysis of ammonia–borane over an efficient NiAgPd/C catalyst. Int. J. Hydrogen Energ. 39 (2014) 20031-20037. [35] B. Zhao, J. Liu, L.Zhou, D. Long, K. Feng, X. Sun, J. Zhong, Probing the electronic structure of M-graphene oxide (M= Ni, Co, NiCo) catalysts for hydrolytic dehydrogenation of ammonia borane, Appl. Surf. Sci. 362 (2016) 79-85. [36] M. Rakap, EE. Kalu, S. Özkar, Polymer-immobilized palladium supported on TiO2 (Pd–PVB–TiO2) as highly active and reusable catalyst for hydrogen generation from the hydrolysis of unstirred ammonia–borane solution. Int. J. Hydrogen Energ. 36 (2011) 1448-1455. [37] N. S. Ciftci, O. Metin, Monodisperse nickel–palladium alloy nanoparticles supported on reduced graphene oxide as highly efficient catalysts for the hydrolytic dehydrogenation of ammonia borane, Int. J. Hydrogen Energ. 39
(2014) 18863-18870. [38] M. Rakap, E. E. Kalu, S. Özkar, Hydrogen generation from the hydrolysis of ammonia borane using cobalt-nickel-phosphorus (Co–Ni–P) catalyst supported on Pd-activated TiO2 by electroless deposition. Int. J. Hydrogen Energ. 36 (2011) 254-261. [39] C. Ruas, D.Fischer, M. Gelesky, PVP-stabilized palladium nanoparticles in silica as effective catalysts for hydrogenation reactions. Int. J. Nanotechnol. (2013) 1-6. [40] X. Gu, Z. Lu, H. Jiang, T. Akita, Q. Xu, Synergistic catalysis of metal-organic framework-immobilized Au-Pd nanoparticles in dehydrogenation of formic a cid for chemical hydrogen storage. J. Am. Chem. Soc.133 (2011) 11822-11825. [41] M. Rakap, S. Ozkar, Hydrogen generation from the hydrolysis of ammonia-borane using intrazeolite cobalt (0) nanoclusters catalyst, Int. J. Hydrogen Energ. 35 (2010) 3341-3346.
S0169-4332(17)32525-4 http://dx.doi.org/10.1016/j.apsusc.2017.08.167 APSUSC 37004
To appear in:
APSUSC
Received date: Revised date: Accepted date:
25-7-2017 16-8-2017 24-8-2017
Please cite this article as: Doudou Gao, Yuhong Zhang, Liqun Zhou, Kunzhou Yang, CuNi NPs supported on MIL-101 as highly active catalysts for the hydrolysis of ammonia borane, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.08.167 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
CuNi NPs supported on MIL-101 as highly active catalysts for the hydrolysis of ammonia borane Doudou Gao, Yuhong Zhang, Liqun Zhou, Kunzhou Yang Hubei 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, China
Graphical abstract
The Scheme of the AB hydrolysis catalyzed by CuxNi1@MIL-101.
Corresponding author. E-mail address: [email protected] (L. Zhou).
Highlights 1. Cu2Ni1@MIL-101 shows good catalytic activity and durability for hydrolysis of AB. 2. There are strong synergistic effects between metals Cu and Ni nanoparticles. 3. The bi-functional effects between CuNi nanoparticles and MIL-101 are significant.
ABSTRACT: The catalysts containing Cu, Ni bi-metallic nanoparticles were successfully synthesized by in-situ reduction of Cu2+ and Ni2+ salts into the highly porous and hydrothermally stable metal-organic framework MIL-101 via a simple liquid impregnation method. When the total amount of loading metal is 3×10-4mol, Cu2Ni1@MIL-101
catalyst
shows
higher
catalytic
activity
comparing
to
CuxNiy@MIL-101 with different molar ratio of Cu and Ni (x, y= 0, 0.5, 1.5, 2, 2.5 ,3). Cu2Ni1@MIL-101 catalyst has the highest catalytic activity comparing to mono-metallic Cu and Ni counterparts and pure bi-metallic CuNi nanoparticles in hydrolytic dehydrogeneration of ammonia borane (AB) at room temperature. Additionally, in the hydrolysis reaction, the Cu2Ni1@MIL- 101 catalyst possesses excellent catalytic performances, which exhibit highly catalytic activity with turn over frequency (TOF) value of 20.9 mol H2 min-1 Cu mol-1 and a very low activation energy value of 32.2 kJ mol-1. The excellent catalytic activity has been successfully achieved thanks to the strong bi-metallic synergistic effects, uniform distribution of nanoparticles and the bi-functional effects between CuNi nanoparticles and the host of MIL-101. Moreover, the catalyst also displays satisfied durable stability after five cycles for the hydrolytically releasing H2 from AB. The non-noble metal catalysts have broad prospects for commercial applications in the field of hydrogen-stored materials due to the low prices and excellent catalytic activity.
Keywords: CuNi@MIL-101; Heterogeneous catalysis; Ammonia-borane; Releasing hydrogen
1. Introduction Hydrogen is an environmentally friendly fuel and regarded as a new generation of the ideal alternative energy sources [1]. Currently, various hydrogen storage approaches have been investigated, including metal hydrides [2], sorbent materials [3], and coordination of hydride systems [4]. In these materials, ammonia borane (NH3BH3, denoted as AB) is nontoxic, stable, environmentally benign, and is considered to be an attractive solid hydrogen storage materials due to its high hydrogen mass capacity, high solubility and demonstrated stability in neutral aqueous solutions [5]. The catalytic hydrolysis reaction can be briefly expressed as follows: NH3 BH 3 2H 2O Cata . NH 4 BO2 3H 2
So far, the optimal compromise between costs, efficiency and recyclability still remains a considerable challenge for the practical use of AB as a potential hydrogen storage material. Up to now, a series of noble metal NPs@MOFs catalysts were tested for hydrolytic dehydrogenation of AB [6]. Moreover, it has very large windows and surface areas with Slangmuir = 3750 m2 g-1, which easily allow the introduction of new species into the cages. These properties, together with high adsorption capacities, make MIL-101 become an attractive candidate for the host of the NPs@MOFs catalysts [7]. Recently, a series of metal NPs@MIL-101 catalysts were tested for the hydrolytic dehydrogenation of AB, such as Pd@MIL-101[8], Ru@MIL-101[9], CuPd/MIL-101, PdNi@MIL-101[10], and RuCuCo@MIL-101 and so on. However, the widespread utilization of noble metal-based catalysts was hindered by their high price and limited abundance, and the research on the catalysts of non-noble metal NPs@MOFs were rarely reported. Especially, the non-noble bi-metallic synergistic effects can improve the catalytic activity. Therefore, the synthesis of catalysts from non-noble bi-metallic nanoparticles supported on MOFs is our purpose. In this work, we report the generation of hydrogen from hydrolysis of AB over non-noble bi-metallic CuNi nanoparticles immobilized in MIL-101 as efficient catalysts at room temperature, as shown in scheme 1. The structure, morphology, size, composition and specific area of the catalysts with different CuNi nanoparticles loading were discussed in detail, the catalytic activity and
influence factors as well as stability were investigated by comparing the catalysts for hydrolysis of AB. In addition, the activation energy (Ea) for the hydrolysis reaction has been measured by different temperature experiment.
Scheme 1. Cu (NO3)2·3H2O and Ni (NO3)2·6H2O were in-situ reduced by NaBH4 and the hydrolytic dehydrogeneration of ammonia borane (AB) at room temperature catalyzed by CuNi@MIL-101.
2. Experimental 2.1. Synthesis of MIL-101 MIL-101 was synthesized according to hydrothermal method [11, 12]. Terephthalic acid (2007.0 mg, 5.0 mmol), Cr(NO3)3·9H2O (823.0 mg, 5.0 mmol), aqueous HF (0.12 mL, 40 wt%) and de-ionized water (24 mL) were placed in a 40 mL Teflon-liner autoclave and heated at 200 °C for 8 h. After natural cooling, the resulting green powder was doubly filtered off using a filter cloth with pore sizes between 40 and 100 um to remove the unreacted terephthalic acid crystals and then purified by a magnetic stirring at 25 °C for 24 h in ethanol and solvothermal treatment in NH4F solution (30 mM) at 60 °C for 10 h. Finally, the purified MIL-101 was dried overnight
at 80 °C in a vacuum drying oven. 2.2. Synthesis of CuNi@MIL-101 and other catalysts 200 mg of MIL-101 was added to a flask containing 30 mL of de-ionized water. Ultrasonication was required to get a uniform dispersion. Next, Ni (NO3)2·6H2O and Cu (NO3)2·3H2O were 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. CuNi@MIL-101 catalyst was collected through filtration and dried in the vacuum oven at 80°C for 12 h. During the synthesis of mono-metal Cu@MIL-101, Ni@MIL-101 and pure CuNi NPs catalysts, the amounts of Cu and Ni were consistent with that of Cu2Ni1@MIL-101. And CuNi NPs was prepared as follows: Cu (NO3)2·3H2O (0.2 M, 1 ml) and Ni (NO3)2·6H2O (0.1 M, 1 ml) 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 NaBH4 (10 mL, 0.013 mol L-1) 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°C overnight in a vacuum oven for further use. The designed condition for the synthesis of different metal loading catalysts is shown in Table 1.
Table 1.Designed conditions for the synthesis of different metal loading catalysts Cu (10-4 mol) Ni (10-4 mol)
2.5 0.5
2 1
1.5 1.5
1 2
0.5 2.5
3 0
0 3
2 0
0 1
2.3. Catalytic performance test The experiment was conducted in a two-necked round bottom flask equipped with a gas burette and a pressure-equalization funnel to introduce 20 mL de-ionized water. In all the experiments, a mixture of 50.0 mg CuxNiy@MIL-101 and 18.5 mg AB were kept in the flask. When the de-ionized water was added to the above mixture, the reaction of AB hydrolysis rapidly started, and the volume of the generated hydrogen gas was recorded by the displacement level of water in the burette at a certain time interval. A
water bath was used to control the temperature of the reaction solution. In order to get the activation energy (Ea), the reaction was carried out at different temperatures (25, 30, 35 and 40°C) with CuNi@MIL-101 catalyst. 2.4. Stability test The durability of Cu2Ni1@MIL-101 catalyst in the hydrolysis of AB was evaluated by performing the recyclability tests at room temperature. In a typical test, 10 mL of Cu2Ni1@MIL-101 (50 mg) suspension solution and 18.5 mg AB (10 mL, 0.6 mmol) were added into the reaction flask at 25°C, and the evolution of gas was measured as described above. After the hydrolysis reaction was completed, another equivalent of AB (18.5 mg) 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°C. In a typical reusability test, 18.5 mg AB (10 mL, 0.6 mmol) was added to 10 mL of Cu2Ni1@MIL-101 (50 mg) suspension solution, the evolution of gas was monitored as
described
above.
After
the
hydrolysis
reaction
was
completed,
the
Cu2Ni1@MIL-101 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 °C. 2.5. Characterization X-ray powder diffractions (XRD) of products were examined on a diffractometer (Bruker D8 Advance, Germany) with Cu Ka (40 kV, 40 mA) radiation (λ=0.1540 nm) to identify the phase purity and crystallinity. The surface area measurements were performed with N2 adsorption-desorption isotherms were obtained at 77 K using a Micromeritics ASAP 2020 instrument. The metal loading of the prepared catalyst was determined by an IRIS Intrepid IIXSP inductively coupling plasma-atomic emission spectrometer (ICP-AES). The electronic states were investigated by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi). The morphology 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. The Fourier transform infrared (FTIR) spectra of the products were recorded in KBr pellets on a spectrometer (FTIR- Spectrum one, USA) in transmission mode ranging from 500 to 4000 cm–1 under ambient conditions.
3. Results and discussion The XRD patterns of the as-prepared MIL-101, Cu@MIL-101, Ni@MIL-101, Cu2Ni1@ MIL-101 and Cu2Ni1@MIL-101 after the fifth run are illustrated in Fig. 1. From the wide-angle XRD in Fig. 1a, it can be seen that the XRD pattern of MIL-101 is in good agreement with that reported by Férey et al [11], confirming that MIL-101 was successfully synthesized. The Cu@MIL-101, Ni@MIL-101 and Cu2Ni1@MIL-101 and Cu2Ni1@ MIL-101 after the fifth run are matched with those of the host MIL-101, indicating that the integrity of the MIL-101 framework is unchanged during the catalyst preparation and catalytic process, and MIL-101 can provide a good 3D framework for CuNi NPs. In addition, as shown in Fig. 1b, the intensity of the diffraction peaks of the host MIL-101 decreases substantially with the metal nanoparticles loading, this result can be explained by the changes in the charge distribution and electrostatic fields as a result of the existence of Cu Ni nanoparticles on the surface or in the cages and interaction of their electrophilic surface with framework atoms [13]. Additionally, it can be found that there was no significant diffraction peak about metal Cu and Ni species in Fig 1. This may be attributed to low metal loading and the ultrafine nanoparticles into the pores of MIL-101[14, 15].
Fig. 1. (a) Wide-angle powder X-ray diffraction patterns (b) low-angle powder X-ray diffraction
patterns
of
samples
MIL-101,
Cu@MIL-101,
Ni@MIL-101,
Cu2Ni1@MIL-101 and Cu2Ni1@MIL-101 after the fifth run
The X-ray photoelectron spectroscopy (XPS) was employed to demonstrate metal electronic states of the Cu2Ni1@MIL-101 catalyst. Fig. 2a shows the characteristics peaks of C 1s, O 1s, Ni 2p and Cu 2p. The Cu 2p signal is shown in Fig. 2b. Peaks with binding energies of 935.1 eV and 954.6 eV respectively [16]. The shake-up features around 943.2 and 962.6 eV for the Cu 2p3/2 and Cu 2p1/2, respectively, are compelling evidence and diagnostic of an open 3d9 shell, corresponding to Cu (II) [17]. The peak
fit of Cu 2p3/2 reveals three peaks at 935.3, 934.9 and 932.8 eV, which are assigned to Cu(0), Cu(II) and Cu(I) states, respectively, and the shake-up satellite peaks observed at 942.4 and 944.2 eV are indicative of Cu(II) [18,19]. Fig. 2c shows the two peaks centered at 856.5 and 873.6 eV, which can be attributed to Ni (0) 2p3/2 and Ni (0) 2p1/2, respectively. Peaks with binding energies of 862.4 and 879.0 eV, can be indexed to Ni (II) 2p3/2 and Ni (II) 2p1/2, respectively, such as NiO and Ni (OH) 2 [20]. These results indicate that Ni (0) and Cu (0) can be oxidised during the XPS sampling [21].
Fig.2. XPS spectra of Cu2Ni1@MIL-101: (a) full spectrum; (b) Cu 2p; (c) Ni 2p The morphologies and loading effect of Cu, Ni and CuNi nanoparticles immobilized in MIL-101 were further characterized by TEM. The TEM images of MIL-101, Cu2Ni1@MIL-101, Cu@MIL-101 and Ni@MIL-101 catalysts were shown in Fig. 3a, b, c, d, respectively. As can be seen in Fig. 3a, before loading the metal NPs, the surface of MIL-101 is smooth without any aggregates. Fig. 3b shows that the CuNi nanoparticles are uniformly loaded on the surface of MIL-101 and no significant aggregate was formed, which is the basis for high performance of the catalyst, the average particle size in the Cu2Ni1@MIL-101 catalyst was about 1.75 nm, as shown in Fig. 3f. Comparing to Fig 3b, in Fig. 3c, it can be seen that most of mono-metallic Cu nanoparticles were agglomerated on the surface of MIL-101, and Fig. 3d shows that the Ni nanoparticles were uniformly loaded on the surface of MIL-101 but with less loading, which reduced effective contact area with reactants and interpreted the low catalytic efficiency of mono-metallic catalysts [11, 22]. The mean particle sizes of the metal nanoparticles in Cu@MIL-101 and Ni@MIL-101 were about 7.0 nm and 3.5 nm, respectively, as shown in Fig. 3g, h. In order to study the catalyst recyclability, the TEM of Cu2Ni1@MIL-101 after five cycles were further characterized, see Fig.3e, which shows that after five cycles, there are still a great number of metal nanoparticles loading on MIL-101 surface. Moreover, the MIL-101 framework is maintained well which is the guarantee of excellent durability.
Fig. 3. TEM images of (a) MIL-101; (b) Cu2Ni1@MIL-101(c) Cu@MIL-101; (d) Ni@MIL-101(e) Cu2Ni1@MIL-101after fifth run; particle size distributions of (f) Cu2Ni1@MIL-101; (g) Cu@MIL-101; (h) Ni@MIL-101.
Fig. 4. N2 sorption isotherms of (a) MIL-101, (b) Cu2Ni1@MIL-101 Fig.4 shows the N2 sorption isotherms of (a) MIL-101 and (b) Cu2Ni1@MIL-101. The Brauner-Emmett-Teller (BET) specific surface area of the MIL-101 was 3170 m2 g-1, which is close to the values reported in the literature [11]. However, after the loading of CuNi nanoparticles, the surface area of MIL-101 dropped significantly from 3170m²g-1 to 1317 m²g-1. This phenomenon fully confirmed that the partial small size nanoparticles embedded in the pore of MIL-101, owing to the pore diameter of MIL-101 is about 2.3 nm [11, 12] and the size of CuNi NPs are in the range of 0.5-3.0 nm. After loading metal NPs, the small size metal nanoparticles would occupy or block the cages of MIL-101 and the big size nanoparticles are mainly deposited on the
surface of MIL-101 and directly lead to a decrease in the specific surface area, which has been proved by previous work [9, 10, 23]. In order to further illustrate the presence of Cu and Ni elements in the catalyst, EDX characterization was also performed, as shown in Fig. 5. The results show that the chemical composition of the product is element Cu, Ni, Cr, C, O. In addition, the actual loading of the metal Cu and Ni in the catalyst Cu2Ni1@MIL-101 was determined to be 3.4 wt% and 1.4 wt% by ICP-AES, which can explain the existence of Cu and Ni, as shown in Table 2. The results show that the actual ratios of Cu: Ni is almost similar to the precursor ratios in the CuxNiy@MIL-101 catalysts.
Fig. 5. EDX spectrum of Cu2Ni1@MIL-101.
Table 2. Elemental analysis of CuXNiy@MIL-101 catalysts determined by ICP-AES. Catalysts
Cu2.5Ni0.5
Cu2Ni1
Cu1.5Ni1.
Cu1Ni2
Cu0.5Ni2.5
5
Precursor ratios Cu:Ni Actual ratios Cu:Ni Actual loading of Cu wt% Actual loading of Ni wt%
5:1 4.3 : 1 2.2 % 0.5 %
2:1 2.3 : 1 3.4 % 1.4 %
1:1 1:1 2.9 % 2.7 %
1:2 1 : 2.1 0.7 % 1.4 %
1:5 1 : 4.5 0.6 % 2.7 %
Fig. 6 shows the hydrogen generation volumes as the function of reaction time obtained by the hydrolysis of AB in the CuxNiy@MIL-101 catalysts. The results are recorded in Fig. 6a. It is evident that, by changing the molar ratios 2.5:0.5, 2:1, 1.5:1.5, 1:2 to 0.5:2.5 of Cu and Ni, the bi-metallic nanoparticles show different catalytic
activities. Research shows that when the total amount of loading metal was 3×10-4mol, the catalyst Cu2Ni1@MIL-101 and Cu1.5Ni1.5@MIL-101 show the similar catalytic activity at the beginning and the end of the reaction. But in the middle stage of the reaction, Cu2Ni1@MIL-101 exhibited higher catalytic activity than Cu1.5Ni1.5@MIL -101, with the turn over frequency (TOF) value of 20.9 mol H2 min-1 Cu mol-1. Comparing to the other catalyst used in the same reaction, it can be seen that as-synthesized Cu2Ni1@MIL-101 catalyst has a higher TOF value and satisfied catalytic activity. In order to further study the synergistic effects among non-noble metal Cu and Ni, we measured the catalytic activity of mono-metallic counterparts catalysts, the results were shown in Fig. 6b, when the same amount of total metal, in comparison with the case of the mono-metallic Cu3@MIL-101 or Ni3@MIL-101 catalyst, a high hydrogen generation rate was observed for the corresponding Cu2Ni1@MIL-101. Moreover, the catalytic performance of the mixture of mono-metallic counterparts Cu2@MIL-101 + Ni1@MIL-101 was obviously inferior to that of the Cu2Ni1@MIL- 101. These results fully suggest that the strong synergistic interaction between Cu and Ni plays an important role in the hydrolysis of NH3BH3. Furthermore, similar synergetic features in bi-metallic NPs were found in our previous work and literatures [23, 24, 25, 26]. For alloy NPs, synergetic effects may be mainly ascribed to electronic effects, which are important when the metal atoms have significant differences in electronegativity [26], and geometric effects. In the present case, the observed synergetic effects in catalysis might be mainly due to the geometric effects, not the electronic effects, because the electronegativities of Cu (1.90) and Ni (1.91) are very close. In the geometric effects, bi-metallic synergistic effects may be related to the metal crystal structure of Cu and Ni NPs, Cu and Ni both are A1-type crystal structure, When forming CuNi alloy NPs, Ni replace some Cu in Cu lattice and keep the crystal system of Cu sustaining the A1-type crystal structure, which makes Cu and Ni exhibit good synergistic effects.
Fig.6. Hydrogen generation from the hydrolysis of AB (a) catalyzed by Cu2.5Ni0.5@MIL-101, Cu2Ni1@MIL-101, Cu1.5Ni1.5@MIL-101, Cu1Ni2@MIL-101 and Cu0.5Ni2.5 @MIL-101; (b) catalyzed by Cu2Ni1@MIL-101, Cu@MIL-101, Ni@MIL-101
and
Cu2@MIL-101
+
Cu2Ni1@MIL-101, MIL-101 and CuNi NPs.
Ni1@MIL-101;
(c)
catalyzed
by
In addition, we conducted a thorough study of the bi-functional effects between CuNi nanoparticles and the host of MIL-101, as shown in Fig. 6c, the catalytic activity of Cu2Ni1@MIL-101 is superior to that of pure Cu2Ni1 nanoparticles. Furthermore, it is obvious that the pure MIL-101 almost have no catalytic activity toward hydrolysis of AB, and the poor catalytic activity has been proved by previous work [24].Those results fully demonstrate that the bi-functional effects between CuNi alloy NPs and MIL-101 significantly enhanced the reaction activity of Cu2Ni1@ MIL-101 catalyst, in which MIL-101 can provide a good 3D framework for CuNi NPs in spite of it does not involved in the hydrolysis reaction of AB. From Fig.6, it can be found that the reaction rate increased at first and then decreased when reached a certain time. This phenomenon may relevant to catalytic mechanism of the CuNi@MIL-101 catalysts for the hydrolysis reaction of AB. For the present heterogeneous catalytic reaction, there seems no doubt that the catalytic reaction takes place on the surface area of the metal catalyst [27]. The mechanism involves two steps. Firstly, AB interacts with the surface of CuNi nanoparticles 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 [28, 29]. In order to get the activation energy (Ea) of the AB hydrolysis catalyzed by Cu2Ni1@MIL-101, the hydrolytic experiments at different temperatures ranged from 25 to 40 °C were carried out. Fig. 7a shows that the hydrolytic rates increase with the increase of reaction temperature, and the values of rate constant k at different temperatures 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. 7b, from which the apparent activation energy (Ea) is determined to be approximately 32.2 kJ mol-1 for AB. By comparing the activation energy and turn over frequency of non-noble metal catalyst Cu2Ni1@MIL-101 with that of noble metal catalysts in Table 3, we can see that the Cu2Ni1@MIL-101 shows the good catalytic activity and the low activation energy. The results indicate that the as-synthesized Cu2Ni1@MIL-101 catalyst is a very promising candidate for the hydrolysis of AB.
Fig. 7. (a) Hydrogen evolution rate of Cu2Ni1@MIL-101 for the hydrolysis of AB at different temperatures; and (b) Arrhenius curves. Additionally, the as-synthesized Cu2Ni1@MIL-101 was tested in terms of durability in five cyclic uses, as shown in Fig. 8. The catalyst still exhibits high catalytic activity after five cycles, retaining 75% of its initial catalytic activity, see Table 3, the Cu2Ni1@MIL-101 has a good durability by comparing with that of various noble metal catalysts, which makes Cu2Ni1@MIL-101 very conducive in the practical application. The high durability should be attributed to the crystalline structure of the catalyst is mostly unchanged and the ultrafine CuNi NPs still effectively immobilized in MIL-101 through the catalytic cycles. In Fig. 9, the intensity and position of FT-IR peaks of MIL-101, initial Cu2Ni1@MIL-101 and Cu2Ni1@MIL-101 after the fifth run
show no obvious change. This phenomenon indicates that the crystalline structure of the host of MIL-101 is very stable, that provides favorable conditions for CuNi NPs effectively immobilized in the framework of MIL-101 after the catalytic cycles [40].
Fig. 8. (a) The time course plots for durability of Cu2Ni1@MIL-101; (b) Percentage of initial catalytic activity of Cu2Ni1@MIL-101 in successive runs after the reuse for the hydrolysis of AB.
Table 3.Various catalyst systems tested in the hydrolysis of AB at room temperature. Catalyst Pt black
Ea (kJ
TOF (mol H2
mol-1
)
min-1 M mol-1 )
Durability
Ref.
-
13.89
-
[30]
Pd@Co/graphene Ag/C/Ni Cu75Pd25/RGO
38.9 45.9
37.5 5.32 29.9
59% (5 runs) 79% (4 runs) -
[31] [32] [33]
CoPd/C Pd10Ni6@MIL-101 2.1 wt% Pd@RGO
27.5 31.7 40
22.7 83.1 26.3
85% (5 runs) 65% (3 runs)
[34] [23] [35]
Pd-PVP-TiO2
55.9
3.1
89% (5 runs)
[36]
rGO-Ni30Pd70 Co-Ni-P/Pd-TiO2 PVP-stabilized Pd Pd/zeolite Cu2Ni1@MIL-101
45 54.7 35 56 32.2
28.7 0.3 22.3 6.25 20.9
63% (3 runs) 69% (5 runs) 65% (5runs) 75% (5 runs)
[37] [38] [39] [21] This study
Fig. 9 FT-IR of (a) MIL-101; (b) initial Cu2Ni1@MIL-101; (c) Cu2Ni1@MIL-101 after the fifth run.
The reduction of catalytic activity may be due to the increasing viscosity of the solution or the deactivation effect of the increasing metaborate concentration during the hydrolysis [22, 41]. Besides, the catalytic activity decrease may be related to the metal
leaching. In Table 4, metal leaching in the five-times used were determined to be 7.7%, 9.7%, 12.1%, 14.8% and 17.6%, respectively. Table 4. The metal leaching of Cu2Ni1@MIL-101 (50mg) in the subsequent runs of the hydrolysis of ammonia borane. Run
Cu leaching (mg)
Ni leaching (mg)
Metal leaching(%)
1 2
0.165 0.207
0.018 0.025
7.7 9.7
3
0.256
0.032
12.1
4 5
0.315 0.375
0.037 0.045
14.8 17.6
4. Conclusions In summary, MIL-101 supported CuNi nanoparticles catalysts have been successfully synthesized, which exhibit highly catalytic activity with turn over frequency (TOF) value of 20.9 mol H2 min-1 Cu mol-1 and the good durability, maintaining 75% after fifth runs. In addition, the activation energy (Ea) was determined to be 32.2 kJ mol-1. The excellent catalytic performances of catalytic hydrolysis of AB were owing to the strong bi-metallic synergistic effects, uniform distribution of nanoparticles as well as bi-functional effects between CuNi NPs and the host of MIL-101. In addition, as the price of noble metals reached their historical height, non noble metal component in catalyst not only improves the catalytic activity, but also reduces the cost which is a key for industry for a sustainable future. Therefore, easy preparation and the high catalytic performances reveal that the CuNi@MIL-101 catalyst is a promising candidate to be employed in developing AB as a highly efficient, portable hydrogen storage system.
Acknowledgments This project was supported by the Natural Science Fund for Creative Research Groups of Hubei Province (2014CFA015), Hubei Province Education Office Key
Laboratory (2016-KL-007) of China. This work was also supported by the Hubei College Students' Innovation Training Program of China (201410512024, 201510 512030).
References [1] S. Ma, J. Xie, J. Wen, K. He, X. Li, W. Liu, X. Zhang, Constructing 2D layered hybrid CdS nanosheets/MoS2 heterojunctions for enhanced visible-light photocatalytic H2 generation. Appl. Surf. Sci. 391 (2017) 580-591. [2] Y. Wan, D. Zhao, On the controllable soft-templating approach to mesoporous silicates, Chem. Rev. 107 (2007) 2821-2860. [3] KSW. Sing, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Recommendations 1984), Plast. Reconstr. Surg. 57 (1985) 603-619. [4] M. E. Bluhm, M. G. Bradley, R. Butterick, U. Kusari, L. G. Sneddon, Amineborane-based chemical hydrogen storage: enhanced ammonia borane dehydrogenation in ionic liquids, J. Am. Chem. Soc. 128 (2006) 7748-7749. [5] B. Peng, J. Chen, Ammonia borane as an efficient and lightweight hydrogen storage medium, Energ. Environ. Sci. 1 (2008) 479-483. [6] J. Shen, L. Yang, K. Hu, W. Luo, G. Cheng, Rh nanoparticles supported on graphene as efficient catalyst for hydrolytic dehydrogenation of amine boranes for chemical hydrogen storage，Int. J. Hydrogen Energ. 40(2015) 1062-1070. [7] R. Fang, H. Liu, R. Luque, Y. Li, Efficient and selective hydrogenation of biomass-derived furfural to cyclopentanone using Ru catalysts, Green Chem. 17 (2015) 4183-4188. [8] Y. Zhao, M. Liu, B. Fan, Y. Chen, W. Lv, N. Lu, R. Li, Pd nanoparticles supported on ZIF-8 as an efficient heterogeneous catalyst for the selective hydrogenation of cinnamaldehyde, Catal. Commun. 57 (2014) 119-123. [9] T. Liu, Q. Wang, B. Yan, M. Zhao, W. Li, H. Bie, Ru nanoparticles supported on MIL-101 by double solvents method as high-performance catalysts for catalytic hydrolysis of ammonia borane, J. Nanomater. 2015 (2015) 679526 (1-5). [10] A. Dhakshinamoorthy, H. Garcia, Catalysis by metal nanoparticles embedded on metal–organic frameworks, Chem. Soc. Rev. 41 (2012) 5262-5284. [11] G. Ferey, C. Mellotdraznieks, C. Serre, F. Millange, J. Dutour, S. Surble, I. Margiolaki, A chromium terephthalate-based solid with unusually large pore volumes and surface area, Science 309 (2005) 2040-2042. [12] D. Hong, H. Wang, YK, C. Serre, G. Ferey, J. Chang, Porous chromium terephthalate MIL‐ 101 with coordinatively unsaturated sites: surface functionalization, encapsulation, sorption and catalysis, Adv. Funct. Mater. 19 (2009) 1537-1552.
[13] T. Loiseau, L. Lecroq, C. Volkringer, J. Marrot, G. Ferey, M. Haouas, F. Taulelle, S. Bourrelly, P. Llewellyn, M. Latroche, MIL-96, a porous aluminum trimesate 3D structure constructed from a hexagonal network of 18-membered rings and mu3-oxo-centered trinuclear units, J. Am. Chem. Soc. 128 (2006) 10223-10230. [14] M. Yurderi, A. Bulut, M. Zahmakiran, M. Gulcan, S. Özkar, Ruthenium(0) nanoparticles stabilized by metal-organic framework (ZIF-8): Highly efficient catalyst for the dehydrogenation of dimethylamine-borane and transfer hydrogenation of unsaturated hydrocarbons using dimethylamine-borane as hydrogen source, Appl. Catal. B: Environ. 160 (2014) 534-541. [15] M. S. El-shall, V. Abdelsayed, A.E.R.S. Khder, H.M.A. Hassan, H.M. El-kaderi, T. E. Reich, Metallic and bimetallic nanocatalysts incorporated into highly porous coordination polymer MIL-101, J. Mater. Chem. 19 (2009) 7625-7631. [16] C. C. Chusuei, M. A.Brookshier, D. W. Goodman, Correlation of relative X-ray photoelectron spectroscopy shake-up intensity with CuO particle size, Langmuir 15 (1999) 2806-2808. [17] D. Bazin, I. Kovacs, JA. Lynch, L. Guczi, Ru-Co/NaY bimetallic catalysts: in situ EXAFS study at Co K- and Ru K-absorption edges, Appl. Catal. A: Gen. 242 (2003) 179-186. [18] G. Wang, B. Huang, Z. Lou, Z. Wang, X. Qin, X. Zhang, Y. Dai, Valence state heterojunction Mn3O4/MnCO3: Photo and thermal synergistic catalyst, Appl. Catal. B: Environ. 180 (2016) 6-12. [19] W. Wang, G. Wang, X. Wang, Y. Zhan, Y. Liu, C. Zheng, Synthesis and characterization of Cu2O nanowires by a novel reduction route, Adv. Mater. 14 (2002) 67-69. [20] M. Zahmakıran, T. Ayvalı, S. Akbayrak, S. Çalışkan, D. Çelik, S. Özkar, Zeolite framework stabilized nickel (0) nanoparticles: active and long-lived catalyst for hydrogen generation from the hydrolysis of ammonia-borane and sodium borohydride, Catal. Today 170 (2011) 76-84. [21] K. B. Hong, C. W. Lee, E. K. Yum, Synthesis of 2-substituted indoles by palladium-catalyzed heteroannulation with Pd-NaY zeolite catalysts. Tetrahedron Lett. 35(2004) 693-697. [22] N. Cao, T. Liu, J. Su, X. Wu, W. Luo, G. Cheng, Ruthenium supported on MIL-101 as an efficient catalyst for hydrogen generation from hydrolysis of amine boranes, New. J. Chem. 38 (2014) 4032-4035. [23] L. Zhang, L. Zhou, K. Yang, D. Gao, C. Huang, Y. Chen, F. Zhang, X. Xiong, L. Li, Q. Xia, Pd Ni nanoparticles supported on MIL-101 as high-performance catalysts for hydrogen generation from ammonia borane, J. Alloy. Compd. 677 (2016) 87-95. [24] Z. H. Lu, J. P. Li, G. Feng, Q. L. Yao, F. Zhang, R. Y. Zhou, D. J. Tao, X. S. Chen, Z. Q. Yu, Synergistic catalysis of MCM-41 immobilized Cu-Ni nanoparticles in hydrolytic dehydrogeneration of ammonia borane, Int. J. Hydrogen Energy 39 (2014) 13389-13395.
[25] 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, J. Alloy. Compd. 694 (2017) 662-671. [26] J. Li, L. Zhu, Q. Xu, Non-noble bimetallic CuCo nanoparticles encapsulated in the pores of metal–organic frameworks: synergetic catalysis in the hydrolysis of ammonia borane for hydrogen generation, Catal Sci Technol. 5(2015) 525-530. [27] D. Ke, Y. Li, J. Wang, L. Zhang, J. Wang, X. Zhao, S. Yang, S. Han, Facile fabrication of poly(amidoamine) (PAMAM) dendrimers-encapsulated Ag–Co bimetallic nanoparticles for highly efficient dehydrogenation of ammonia Borane, Int. J. Hydrogen. Energ. 41 (2016) 2564-2574. [28] Q. Xu, M. Chandra, Catalytic activities of non-noble metals for hydrogen generation from aqueous ammonia–borane at room temperature, J. Power Sources 163 (2006) 364-370. [29] G. Guella, C. Zanchetta, B.R. Patton, A. Miotello, New insights on the mechanism of palladium-catalyzed hydrolysis of sodium borohydride from 11B NMR measurements, J. Phys. Chem. B 110 (2006) 17024-17033. [30] H. Liang, G. Chen, S. Desinan, R. Rosei, F. Rosei, D. Ma, In situ facile synthesis of ruthenium nanocluster catalyst supported on carbon black for hydrogen generation from the hydrolysis of ammonia-borane, Int. J. Hydrogen Energ. 37 (2012) 17921-17927. [31] N. Cao, J. Su, W. Luo, G. Cheng, Hydrolytic dehydrogenation of ammonia borane and methylamine borane catalyzed by graphene supported Ru @Ni core–shellnanoparticles, Int. J. Hydrogen Energ. 39 (2014) 426-435. [32] M. Wen, B. Sun, B. Zhou, Q. Wu, J. Peng, Controllable assembly of Ag/C/Ni magnetic nanocables and its low activation energy dehydrogenation catalysis. J. Mater. Chem. 22 (2012) 11988-11993. [33] K. Güngörmez, Ö. Metin, Composition-controlled catalysis of reduced graphene oxide supported CuPd alloy nanoparticles in the hydrolytic dehydrogenation of ammonia borane. Appl. Catal. A-Gen. 494 (2015) 22-28. [34] L. Hu, B. Zheng, Z. Lai, K-W. Huang, Room temperature hydrogen generation from hydrolysis of ammonia–borane over an efficient NiAgPd/C catalyst. Int. J. Hydrogen Energ. 39 (2014) 20031-20037. [35] B. Zhao, J. Liu, L.Zhou, D. Long, K. Feng, X. Sun, J. Zhong, Probing the electronic structure of M-graphene oxide (M= Ni, Co, NiCo) catalysts for hydrolytic dehydrogenation of ammonia borane, Appl. Surf. Sci. 362 (2016) 79-85. [36] M. Rakap, EE. Kalu, S. Özkar, Polymer-immobilized palladium supported on TiO2 (Pd–PVB–TiO2) as highly active and reusable catalyst for hydrogen generation from the hydrolysis of unstirred ammonia–borane solution. Int. J. Hydrogen Energ. 36 (2011) 1448-1455. [37] N. S. Ciftci, O. Metin, Monodisperse nickel–palladium alloy nanoparticles supported on reduced graphene oxide as highly efficient catalysts for the hydrolytic dehydrogenation of ammonia borane, Int. J. Hydrogen Energ. 39
(2014) 18863-18870. [38] M. Rakap, E. E. Kalu, S. Özkar, Hydrogen generation from the hydrolysis of ammonia borane using cobalt-nickel-phosphorus (Co–Ni–P) catalyst supported on Pd-activated TiO2 by electroless deposition. Int. J. Hydrogen Energ. 36 (2011) 254-261. [39] C. Ruas, D.Fischer, M. Gelesky, PVP-stabilized palladium nanoparticles in silica as effective catalysts for hydrogenation reactions. Int. J. Nanotechnol. (2013) 1-6. [40] X. Gu, Z. Lu, H. Jiang, T. Akita, Q. Xu, Synergistic catalysis of metal-organic framework-immobilized Au-Pd nanoparticles in dehydrogenation of formic a cid for chemical hydrogen storage. J. Am. Chem. Soc.133 (2011) 11822-11825. [41] M. Rakap, S. Ozkar, Hydrogen generation from the hydrolysis of ammonia-borane using intrazeolite cobalt (0) nanoclusters catalyst, Int. J. Hydrogen Energ. 35 (2010) 3341-3346.