Synergetic Catalysis of Non-noble Bimetallic Cu–Co Nanoparticles

Article pubs.acs.org/JPCC

Synergetic Catalysis of Non-noble Bimetallic Cu−Co Nanoparticles Embedded in SiO2 Nanospheres in Hydrolytic Dehydrogenation of Ammonia Borane Qilu Yao,† Zhang-Hui Lu,*,† Yuqing Wang,† Xiangshu Chen,*,† and Gang Feng‡ †

Jiangxi Inorganic Membrane Materials Engineering Research Centre, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, China ‡ Shanghai Research Institute of Petrochemical Technology SINOPEC, Shanghai 201208, China S Supporting Information *

ABSTRACT: Ultrafine non-noble bimetallic Cu−Co nanoparticles (∼2 nm) encapsulated within SiO2 nanospheres (Cu−Co@ SiO2) have been successfully synthesized via a one-pot synthetic route in a reverse micelle system and characterized by SEM, TEM, EDS, XPS, PXRD, ICP, and N2 adsorption−desorption methods. In each core−shell Cu−Co@SiO2 nanosphere, several Cu−Co NPs are separately embedded in SiO2. Compared with their monometallic counterparts, the bimetallic core−shell nanospheres CuxCo1−x@SiO2 with different metal compositions show a higher catalytic performance for hydrogen generation from the hydrolysis of ammonia borane (NH3BH3, AB) at room temperature, due to the strain and ligand effects on the modification of the surface electronic structure and chemical properties of Cu−Co NPs in the SiO2 nanospheres. Especially, the Cu0.5Co0.5@SiO2 nanospheres show the best catalytic performance among all the synthesized CuxCo1−x@SiO2 catalysts in the hydrolytic dehydrogenation of AB. In addition, the activation energy (Ea) of Cu0.5Co0.5@SiO2 core−shell structured nanospheres for the hydrolysis of AB is estimated to be 24 ± 2 kJ mol−1, relatively low values among the bimetallic catalysts reported for the same reaction. Furthermore, the multi-recycle test shows that the bimetallic Cu0.5Co0.5@SiO2 core−shell nanospheres are still highly active for hydrolytic dehydrogenation of AB even after 10 runs, implying a good recycling stability in the catalytic reaction.

1. INTRODUCTION

conditions seems to be the most convenient route for the portable applications (eq 1).12−17

With increasing severeness of the energy crisis and environmental pollution, hydrogen has been considered as one of most ideal energy carriers to satisfy the increasing demand for a sustainable and clean energy supply.1 Safe and efficient storage of hydrogen is very important on the way to the “hydrogen economy”. Currently, numerous hydrogen storage materials have been studied, including sorbent materials, metal hydrides, and chemical hydride systems.2−4 Among these reported hydrogen storage materials, AB is considered as a promising candidate for on-board hydrogen applications because of its high hydrogen content (19.6 wt %), which exceeds gasoline, and low molecular weight and environmentally friendly nature.5−11 The hydrogen stored in AB can be released through different ways, but the hydrolysis of AB by an appropriate catalyst under mild © 2015 American Chemical Society

NH3BH3(aq) + 2H 2O(l) → NH+4 (aq) + BO−2 (aq) + 3H 2(g)

(1)

A wide range of catalysts based mostly on transition-metal nanoparticles (NPs) have been developed for the hydrolytic dehydrogenation of AB.18−27 However, its catalytic hydrolysis also relies heavily on noble metal catalysts.16,18,28,29 For practical use, the development of noble-metal-free catalysts to further improve the kinetic properties is quite an urgent issue to be addressed. Bimetallic nanocatalysts usually have higher catalytic Received: March 12, 2015 Revised: May 14, 2015 Published: June 4, 2015 14167

DOI: 10.1021/acs.jpcc.5b02403 J. Phys. Chem. C 2015, 119, 14167−14174

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Figure 1. Schematic illustration of the formation of M@SiO2 core−shell nanospheres.

Reagent), and sodium borohydride (NaBH4, SB, Aldrich, 99%), were used as received. Ultrapure water with the specific resistance of 18.3 MΩ·cm was obtained by reversed osmosis, followed by ion-exchange and filtration. 2.2. Characterization. Scanning electron microscopy (SEM, Hitachi SU8020), transmission electron microscopy (TEM, JEM-2010), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) equipped with an energy-dispersive X-ray spectroscopy (EDS) and selected area electron diffraction (SAED) were applied for the detailed microstructure and composition information on the synthesized core−shell nanospheres. X-ray photoelectron spectrometry (XPS) analyses were measured on an ESCALABMKLL X-ray photoelectron spectrometer using an Al Kα source. Powder Xray diffraction (PXRD) was performed on a Rigaku RINT-2200 X-ray diffractometer (Cu Kα). The N2 sorption isotherms were taken by using automatic volumetric adsorption equipment (Belsorp mini II). Both metal content and Cu/Co composition of the synthesized samples were analyzed by inductively couple plasma mass spectroscopy (ICP-MS, Agilent 7500CE). The thermal stability of the sample was measured using a PerkinElmer Diamond thermogravimetric analyzer (TGA) with a heating rate of 10 °C min−1 under an Ar atmosphere. 2.3. Catalysts Preparation. A series of CuxCo1−x@SiO2 (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) core−shell nanospheres with different Cu and Co compositions were prepared using a one-pot synthetic route by a reverse micelle technique according to a literature procedure.46 In a typical synthesis, 2.16 mL of aqueous CuCl2·2H2O and/or Co(NH3)6Cl3 solution with the desired concentrations were mixed with 480 mL of NP-5 (20.16 g) cyclohexane solution. After stirring at room temperature for 15 h, an aqueous solution of ammonia (28 wt %, 2.16 mL) was injected under continuous stirring for 2 h. Then, TEOS (2.49 mL) was added rapidly, and the stirring of the mixture was continued for 2 days at room temperature. The products were isolated by addition of methanol to destabilize the reverse micelle system and collected by centrifugation. The products were further washed with cyclohexane and acetone for several times, after which the CuxCo1−x@SiO2 core−shell nanospheres were obtained. The catalysts of CuxCo1−x@SiO2 with different compositions (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) were determined by ICP-AES. The metal content of the product (Cu + Co)/(Cu + Co + SiO2) was kept at about 4.8 wt %. 2.4. Catalytic Activity Measurement. In general, a mixture of AB (34.3 mg) and catalyst (catalyst/AB = 0.08) was placed in a 50 mL two-neck round-bottomed flask. One neck of the flask was connected to a gas buret to measure the released gas.15 The catalytic reaction was started once the 5 mL of water was injected into the flask with vigorous stirring. The volume of the gas

performance than their monometallic counterparts, owing to the synergistic structural and electronic effects of the bimetallic NPs induced by the so-called strain and ligand effects.30−41 To be active in the catalytic reaction, metal catalysts are required to have an ultrafine and uniform size, which causes the increase in the number of active surface sites. However, most of these catalysts in nanometer size are easily aggregated to large particles during the catalytic processes, resulting in the heavy loss of catalytic active sites and serious catalytic degradation. Hence, it is thus crucial to develop protection strategies to stabilize the metal NPs against agglomeration during the catalytic reaction. In general, capping of metal NPs with organic reagents or inorganic materials is a promising strategy to prevent metal NPs from agglomeration.42−44 However, organic capping reagents usually lack the long-term stability under harsh reaction conditions. From the viewpoint of chemical stability, metal NPs coated with inorganic materials are more useful than those based on organic reagents. Recently, metal NPs within porous SiO2 shells have received much attention due to the possibility of obtaining monodisperse particles in nanometer size.43,45−53 The obtained core−shell structure can sufficiently hinder the metal NPs from aggregation through the protection of porous SiO2 shells, which can increase the stability of metal NPs and enhance the properties of the metal NPs for long-time use. Herein, we present a facile one-pot synthetic route for preparing non-noble bimetallic Cu−Co nanoparticles within porous SiO2 nanospheres (Cu−Co@SiO2) in a reverse micelle system by modifying the previous procedure.46 The synthesized bimetallic core−shell nanospheres Cu−Co@SiO2 exhibit higher catalytic activity, in comparison to their monometallic counterparts, for the hydrolytic dehydrogenation of AB at room temperature. Especially, Cu0.5Co0.5@SiO2 shows the best catalytic activities among the CuxCo1−x@SiO2 system. The effect of reaction temperature, recycle, and thermal stability of catalysts for the hydrolysis of AB have also been studied.

2. EXPERIMENTAL SECTION 2.1. Materials. All chemical reagents were obtained from commercial suppliers and used without further purification. Ammonia borane (NH3BH3, AB, 90%, Aldrich), hexamminecobalt(III) chloride (Co(NH3)6Cl3, >99%, TCI), copper chloride dihydrate (CuCl2·2H2O, >99%, Sinopharm Chemical Reagent Co. Ltd.), polyethylene glycol mono-4-nonylphenyl ether (HO(CHCH2O)nC6H4C9H19 n ≈ 5, NP-5, TCI), cyclohexane (C6H12, >99.5%, Tianjin Fuchen Chemical Reagent), ammonia solution (NH3·H2O, 28%, Nanchang Chemical Works), tetraethoxysilane (Si(OC2H5)4, TEOS, 98%, Aldrich), acetone ((CH3)2CO, >99.5%, Nanchang Chemical Works), methanol (CH3OH, 99.5%, Tianjin Fuchen Chemical 14168

DOI: 10.1021/acs.jpcc.5b02403 J. Phys. Chem. C 2015, 119, 14167−14174

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The Journal of Physical Chemistry C released during the catalytic reaction was monitored by recording the displacement of water in the gas buret. The reaction was finished when there was no more gas evolved. The catalytic reactions were carried out at different temperatures ranging from 298 to 318 K under an ambient atmosphere. 2.5. Recycle/Stability Test. After the hydrolysis reaction was completed, the Cu0.5Co0.5 @SiO2 nanospheres were separated by centrifugation, washed with water, and redispersed in 5 mL of water in a flask. Then, the same amount of AB (34.3 mg) was subsequently added to the reaction system to initiate the reaction. Such a recycle test of the catalysts for the hydrolysis of AB was carried out for 10 runs under an ambient atmosphere at room temperature. In order to test the thermal stability of the Cu0.5Co0.5@SiO2 nanospheres, the synthesized sample was first calcined at 773 K for 2 h under an Ar atmosphere (Cu0.5Co0.5@ SiO2-773 K) and then was used for the dehydrogenation of AB.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of the Catalysts. Bimetallic Cu−Co NPs embedded in SiO2 nanospheres were prepared by using a one-pot synthetic route in a reverse micelle system. The formation of Cu−Co@SiO2 core−shell nanospheres for the hydrolysis of AB is illustrated in Figure 1. In brief, the copper and/or cobalt precursors were simply introduced by adding the aqueous solution in the step of water-in-oil (w/o) reverse micelle formation. As shown in Figure 1, the aqueous metal precursor solution could be contained in micelle particles. When TEOS was injected into the mixture solution, the hydrolysis of TEOS by aqueous ammonia would proceed on the interface between water and oil, resulting in the metal precursor embedded in the SiO2 shells.47 The obtained samples were reduced by AB at room temperature. When AB solution is added to the suspension of metal precursor embedded in the SiO2 shells, both reduction and hydrogen release from the hydrolysis of AB occur concomitantly. After reaction, the samples were characterized by SEM, TEM, EDS, PXRD, XPS, and N2 adsorption−desorption techniques. Figure 2a displays the scanning electron microscopy (SEM) image of the Cu0.5Co0.5@SiO2 sample after reaction. The SEM image shows that the size of Cu0.5Co0.5@SiO2 nanospheres prepared by our one-pot method is quite uniform with an average diameter of about 25 nm. To better understand the core−shell structure, the obtained sample can be further confirmed by transmission electron microscopy (TEM) (Figure 2b,d). TEM images show that bimetallic Cu−Co NPs (dark spots in Figure 2d) with small sizes (∼2 nm) are effectively encapsulated within the well-proportioned spherical SiO2 (∼25 nm), which is confirmed by the high-angle annular dark-field scanning transmission electron microscopy (HADDF-STEM) (Figure 2c). The HAADF-STEM images of Cu@SiO2 and Co@SiO2 in Figure S1 (Supporting Information) also show the ultrafine monometallic Cu and Co NPs inside the SiO2 shells, respectively. The energy-dispersive X-ray spectroscopy (EDS) analyses in Figure 3 for the selected area in the HAADF-STEM image (Figure 2c) shows the Kα peaks corresponding to O (0.53 keV), Si (1.74 keV), Cu (0.93, 8.03 keV), and Co (0.78, 6.93 keV) elements, and Ni signals (7.48, 8.27 keV) from the TEM grids. The EDS analysis implies that bimetallic Cu−Co NPs cores are embedded within the SiO2 shells. Additionally, the surface nature of the Cu0.5Co0.5@SiO2 nanospheres was further characterized by X-ray photoelectron spectroscopy (XPS) (Figure S2, Supporting Information). Figure S2a shows the XPS peaks of Cu 2p of the catalysts. The

Figure 2. Representative (a) SEM, (b, d) TEM, and (c) HAADF-STEM images of the Cu0.5Co0.5@SiO2 core−shell nanospheres.

Figure 3. EDS pattern for the Cu0.5Co0.5@SiO2 core−shell nanospheres.

observed Cu 2p3/2 and Cu 2p1/2 with binding energies of peaks at 933.1 and 952.6 eV correspond to zerovalent Cu, while the two peaks at 935.3 and 954.8 eV stand for oxidized Cu. Figure S2b shows the peaks of Co 2p. There are two peaks at 778.6 and 794.0 eV, which are assigned to 3p3/2 and 3p1/2 of zerovalent Co; the other peaks at 781.4, 785.6, 796.9, and 802.0 eV stand for oxidized Co.25,54 The formation of the oxidized Cu and oxidized Co most likely occurs from sampling during the exposure of samples to air. Powder X-ray diffraction (PXRD) profiles for the CuxCo1−x@ SiO2 nanospheres with different copper contents are shown in Figure 4 and Figure S3 (Supporting Information). The broad diffraction patterns in the range of 2θ = 15−35° are observed in all catalysts, which can be assigned to amorphous SiO2. No diffraction line assignable to any copper and/or cobalt species (Figure 4 and Figure S3) are shown in the PXRD patterns for the samples, probably due to the low metal loading and/or the amorphous phase of metal NPs, which can be evidenced by the corresponding SAED result (Figure S4, Supporting Informa14169

DOI: 10.1021/acs.jpcc.5b02403 J. Phys. Chem. C 2015, 119, 14167−14174

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been applied as catalysts for the hydrolytic dehydrogenation of AB. As shown in Figure 6a, Cu@SiO2 nanospheres exhibit a certain activity in the hydrolysis of AB, whereas Co@SiO2 nanospheres show a very low catalytic activity (inset of Figure 6a) for this reaction at room temperature. Impressively, the Cu0.5Co0.5@SiO2 catalysts exhibit a much higher catalytic activity than their monometallic counterparts for hydrolysis of AB. Figure 6b shows the composition effect of CuxCo1−x@SiO2 (x = 0, 0.2, 0.4, 0.5, 0.6, 0.7, 0.8, 1.0) toward the hydrolytic dehydrogenation of AB. It is clear that the catalytic activity toward the hydrolytic dehydrogenation of AB is correlated with the composition of the CuxCo1−x@SiO2 catalysts. The reaction time decreases at first and then increases with the increase of Cu composition (x value). Obviously, the hydrogen generation rates in the presence of all the bimetallic Cu−Co@SiO2 nanospheres are significantly improved with respect to both monometallic Cu@SiO2 and Co@SiO2. The Cu(II) can be reduced to Cu(0) by AB; however, it is difficult to reduce Co(NH3)6(III) to Co(0) due to the lower reduction potentials of Co(NH3)6(III)/Co(0) (reduction potentials: E0 Cu(II)/Cu(0) = +0.34 eV vs SHE; E0 [Co(NH3)6(III)]/Co(0) = −0.25 eV vs SHE). As displayed in Figure 6, without Cu addition, the Co(NH3)6(III) precursor cannot be easily reduced to Co(0) by using AB as a reducing agent in the present reaction condition, resulting in a very low catalytic activity for the hydrolytic dehydrogenation of AB. Compared to pure Co, the presence of Cu can significantly improve the catalytic activity. The initially formed Cu(0) NPs are active for catalytic dehydrogenation of AB and then generated active intermediate Cu−H species, which might promote the reduction of [Co(NH3)6(III)] to Co(0). Therefore, the Cu component in the CuxCo1−x@SiO2 catalyst not only improves the catalytic activity but also plays a role in the activation of CuxCo1−x@SiO2. For example, when the Cu molar ratio (x value) increases from 0 to 0.2, the reaction time sharply decreases from 595 to 19.2 min. In addition, with the increase in Cu molar ratio, the activity change of the CuxCo1−x@SiO2 system is a Vlike shape, where the best Cu content (x value) is found to be 0.5. The Cu0.5Co0.5@SiO2 nanospheres show the best catalytic performance in the hydrolytic dehydrogenation of AB, generating a stoichiometric amount of hydrogen (H2/AB = 3.0) in the shortest time (8.8 min) among all the CuxCo1−x@ SiO2 catalysts (Figure 6 and Table S1, Supporting Information). The catalytic activities of all the bimetallic Cu−Co@SiO2 core− shell nanospheres are superior to those of their monometallic Cu@SiO2 and Co@SiO2 core−shell nanospheres (Figure 6). The modification of the surface electronic structure and chemical properties of Cu−Co NPs through the strain and ligand effects between Cu and Co are responsible for the high activity of the bimetallic CuCo@SiO2 nanospheres. Similar features in bimetallic NPs were reported in previous works, such as Pt− Ni,30,31,41 Au−Co,32,46 Au−Ni,49 and Cu−M (M = Fe, Co, Ni).34 3.3. Activation Energy for Hydrolytic Dehydrogenation of AB. In order to study the effect of the temperature on the hydrolysis of AB catalyzed by Cu0.5Co0.5@SiO2 nanospheres, a series of experiments were taken at different reaction temperatures ranging from 298 to 318 K. Figure 7a shows the plots of mol (H2/AB) vs reaction time in the hydrolysis of AB (0.2 M, 5 mL) catalyzed by Cu0.5Co0.5@SiO2 nanospheres at various temperatures. The hydrogen generation rate increases by increasing the reaction temperature as expected. The values of rate constant k were measured from the linear portion of each plot in Figure 7a. The Arrhenius plot of ln k versus 1/T for the catalyst is plotted in Figure 7b. The activation energy (Ea) of

Figure 4. PXRD patterns for the Cu@SiO2, Co@SiO2, and Cu0.5Co0.5@ SiO2 core−shell nanospheres.

tion). For free CuCo NPs, CuCo@SiO2 nanospheres with a higher metal loading (20 wt %), and CuCo@SiO2 nanospheres (4.8 wt %) after heat treatment at 773 K, the diffraction peaks assigned to Cu (JCPDS: 04-0836) can be detected, indicating that these metal NPs are crystalline. However, no diffractions of Co are observed in these samples (Figure S5, Supporting Information). The N2 adsorption−desorption was performed at 77 K (Figure 5). For the samples, the BET surface areas are measured

Figure 5. Nitrogen adsorption−desorption isotherms of the Cu@SiO2, Co@SiO2, and Cu0.5Co0.5@SiO2 core−shell nanospheres.

to be 78.6, 85.2, and 107.0 m2 g−1, respectively, for the Cu@SiO2, Co@SiO 2 , and CuCo@SiO 2 nanospheres. From the N 2 adsorption−desorption isotherms, we can see that the Cu@ SiO2, Co@SiO2, and CuCu@SiO2 nanospheres present similar curve shapes. The type-IV curves with a small hysteresis loop occur at a relative pressure of 0.8−0.9, indicating the existence of mesopores in the samples, as evidenced by pore diameter distribution in Figure S6 (Supporting Information). These pores can allow the reactant molecules to directly access the metal NPs cores, and the product molecules can also easily exit through the pores. 3.2. Hydrolytic Dehydrogenation of AB Catalyzed by the CuxCo1−x@SiO2 Nanospheres. To investigate the catalytic performances, the synthesized CuxCo1−x@SiO2 core−shell nanospheres with different Cu compositions (x value) have 14170

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Figure 6. Hydrogen generation from the hydrolysis of aqueous AB (0.2 M, 5 mL) catalyzed by (a) Cu0.5Co0.5@SiO2, Cu@SiO2, Co@SiO2 (and inset of a), and (b) CuxCo1−x@SiO2 core−shell nanospheres with different x values under an ambient atmosphere at 298 K (catalyst/AB = 0.08).

Figure 7. (a) Hydrogen generation from the hydrolysis of aqueous AB (0.2 M, 5 mL) catalyzed by Cu0.5Co0.5@SiO2 core−shell nanospheres at 298−318 K (catalyst/AB = 0.08). (b) Arrhenius plot: ln k versus 1/T.

Cu0.5Co0.5@SiO2 core−shell nanospheres for the hydrolysis of AB is calculated to be 24 ± 2 kJ mol−1, which is lower than most of those reported both of noble and non-noble bimetal catalysts for the same hydrolysis reaction (Table 1).30,31,37−39,52−62 The low activation energy of Cu0.5Co0.5@SiO2 nanospheres for the hydrolysis of AB could be due to the amorphous structure of Cu0.5Co0.5 NPs in the SiO2 nanospheres. The activation energy for CuxCo1−x@SiO2 (x = 0, 0.2 0.4, 0.6, 0.8, 1.0) nanospheres was also studied. The relationship of the Ea values and the content of Cu(x) is shown in Figure S8 and Table S1 (Supporting Information). In the beginning, the Ea value decreases obviously with the increasing content of Cu(x) in the CuxCo1−x@SiO2 (x = 0, 0.2 0.4, 0.5, 0.6, 0.8, 1.0) nanospheres, and then increases slightly. Moreover, with the increase in Cu molar ratio, the Ea value for the CuCo system is also in the shape of a “V”, in line with catalytic activity tests for the hydrolysis of AB. 3.4. Recycle/Stability Test of Cu0.5Co0.5@SiO2 for Hydrolytic Dehydrogenation of AB. The stability of catalyst is greatly important for the practical application. In the present study, the recyclability of Cu0.5Co0.5@SiO2 nanospheres was tested using the catalyst separated from the reaction solution after a previous run of hydrolysis of AB. Such recycle experiments were repeated for 10 runs under an ambient atmosphere at room temperature. Figure 8 shows the activity percentage retained in subsequent AB hydrolysis in the presence of Cu0.5Co0.5@SiO2.

Table 1. Activation Energy (Ea) Values for the Hydrolysis of AB Catalyzed by Different Bimetallic Catalysts catalyst

Ea (kJ/mol)

ref

PtRu@PVP NiAg/C [email protected]/rGO NiCo hexagonal nanoplates PdRh@PVP Ni@Ru Cu0.33Fe0.67 [email protected]/C CoNi/RGO [email protected]/graphene Pt0.65Ni0.35 Cu0.5Ni0.5-400-NC Cu0.2Ni0.8/MCM-41 Ni−Ru alloy Ni0.88Pt0.12 hollow nanospheres NiCu nanorods@C nanofibers Cu0.5Co0.5@SiO2 Pd@Co@MIL-101

56.3 ± 2 51.5 51.3 49.4 46.1 ± 2 44 43.2 41.5 39.83 39.33 39 39 38 37.18 30 28.9 24 ± 2 22

55 56 57 58 59 38 37 60 54 61 31 62 39 38 30 63 this work 64

As shown in Figure 8, the recycle test reveals that the Cu0.5Co0.5@SiO2 nanospheres still remained highly active in 14171

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synthetic route in a reverse micelle system. Small-sized bimetallic Cu−Co NPs (∼2 nm) were effectively embedded in the center of SiO2 nanospheres, confirmed by SEM, TEM, and EDS. The synthesized bimetallic Cu−Co@SiO2 nanospheres exhibit superior catalytic activities than that of their monometallic counterparts for the hydrolysis of AB under an ambient atmosphere at room temperature. Especially, the Cu0.5Co0.5@ SiO2 nanospheres show the highest catalytic activities among all the catalysts with different metal compositions. Moreover, the activation energy for the Cu0.5Co0.5@SiO2 nanospheres was measured to be about 24 ± 2 kJ mol−1, which is lower than that of most of the reported bimetallic nanocatalysts for the hydrolysis of AB. Furthermore, the recyclability test reveals that the bimetallic Cu0.5Co0.5@SiO2 core−shell NPs preserve 93% of their initial catalytic activity even after 10 runs, indicating that the catalysts have good stability in the hydrolysis of AB. The catalyst, which has low cost, high efficiency, and long durability, may strongly encourage the practical application of AB as a sustainable hydrogen storage material.

Figure 8. Catalytic activity versus the number of catalytic runs for the hydrolysis of aqueous AB (0.2 M, 5 mL) catalyzed by Cu0.5Co0.5@SiO2 core−shell nanospheres (catalyst/AB = 0.08).

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the hydrolysis of AB. The catalyst retains 93% of its initial catalytic activity with a complete release of hydrogen by the hydrolysis of AB even after 10 runs. After the recycle test, the bimetallic Cu0.5Co0.5@SiO2 nanospheres were characterized by TEM measurement (Figure 9). The shape and size of the Cu0.5Co0.5@SiO2 nanospheres show no apparent change as compared with that of the sample before the recycle test (Figure 2). In addition, the Cu0.5Co0.5@SiO2 nanospheres exhibit excellent activity even after calcination at high temperature (773 K), which generates a stoichiometric amount of hydrogen (H2/AB = 3.0) in 10.0 min (Figure S9, Supporting Information). The activity test shows that the Cu0.5Co0.5@SiO2 core−shell nanospheres also have good thermal stability, which is further confirmed by the TEM (Figure S4) and PXRD results (Figure S5). These results reveal that the SiO2 nanospheres could completely prevent metal NPs from growth and/or aggregation during the catalytic reaction; thus, the catalyst shows long durability and good thermal stability.

ASSOCIATED CONTENT

S Supporting Information *

The characterization data, and the results of catalytic hydrolysis of AB. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcc.5b02403.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.-H.L.). *E-mail: [email protected] (X.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 21463012 and 21103074) and the Scientific Research Foundation of Graduate School of Jiangxi Province. Z.-H.L. was supported by the Young Scientist Foundation of Jiangxi Province (20133BCB23011), and “Ganpo talent 555” Project of Jiangxi Province.

4. CONCLUSIONS In summary, non-noble bimetallic Cu−Co@SiO2 core−shell nanospheres have been successfully prepared by using a one-pot

Figure 9. Representative TEM images of the Cu0.5Co0.5@SiO2 core−shell nanospheres after the recycle test of AB. 14172

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