Highly Efficient Pt Decorated CoCu Bimetallic Nanoparticles Protected

Dec 13, 2016 - Pt decorated CoCu bimetallic nanoparticles (NPs) coated by silica (Pt-CoCu@SiO2) were synthesized for efficient catalytic hydrogen ...
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Research Article pubs.acs.org/journal/ascecg

Highly Efficient Pt Decorated CoCu Bimetallic Nanoparticles Protected in Silica for Hydrogen Production from Ammonia−Borane Yuzhen Ge, Zameer Hussain Shah, Xi-Jie Lin, Rongwen Lu,* Zhangming Liao, and Shufen Zhang State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, People’s Republic of China

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S Supporting Information *

ABSTRACT: Pt decorated CoCu bimetallic nanoparticles (NPs) coated by silica (Pt-CoCu@SiO2) were synthesized for efficient catalytic hydrogen production from hydrolysis of ammonia−borane (NH3·BH3; AB). Initially, silica-coated CoCu bimetallic NPs (CoCu@SiO2) were prepared by a modified co-reduction method from reverse microemulsion. Further, Pt-CoCu@SiO2 was obtained by a facile spontaneous displacement reaction by using CoCu@SiO2 and H2PtCl6 as the starting materials. The catalytic activity of Pt on bimetallic support was compared to that of monometallic supports, i.e., Co and Cu. The results of catalytic experiments showed that the support of CoCu bimetal can significantly enhance the activity of Pt as compared to that of pure Cu or Co. An impressive turnover frequency (TOF) value of 272.8 molH2 molPt−1 min−1 was achieved at the hydrolysis temperature of 30 °C for Pt-CoCu@SiO2. The detailed formation process of catalysts was described, and the samples were characterized by TEM, STEM, XPS, EDS element mapping, etc. KEYWORDS: Hydrogen production, Ammonia−borane, Hydrolysis, Pt decorating, CoCu bimetal, Reverse microemulsion

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INTRODUCTION Hydrogen is considered one of the cleanest fuels with high chemical energy (142 MJ kg−1).1−3 Recent advancements in catalytic (electro- as well as photo-) hydrogen production are steadily heading toward the commercial use of hydrogen as the sustainable energy carrier.4−6 However, the storage and transport of H2 is still a challenging task for the future “hydrogen economy” since H2 is a gas under ambient conditions and needs high pressure to be converted to compressed H2 (40 g/L at 700 bar) or low temperature to be liquefied H2 (70 g/L at 20 K).7,8 Ammonia−borane (NH3· BH3; AB) has been considered as an intriguing chemical material for hydrogen storage owing to its high hydrogen content (19.6 wt %), low molecular weight (30.87 g mol−1), and easy storage as well as transportation as it is a highly stable solid under ambient conditions.1,9 More importantly, the hydrolysis of AB (NH3BH3 + 2H2O → NH4BO2 + 3H2) can release as much as 3 mol of H2 per mole of AB in the presence of a suitable catalyst.10 Generally, Ru,11,12 Pt,13−17 and Pd,18 etc., have been demonstrated to be highly efficient metals for the catalytic H2 evolution from hydrolysis of AB. However, the relatively low abundance of these noble metals is the bottleneck to extend their practical applications. To solve this problem, one effective strategy is achieving the highest activity among the earth-scarce noble metals. There are two important approaches to improve the catalytic activity of a metallic catalyst. First, one can reduce the particle size of the catalyst to increase the surface-to-volume © 2016 American Chemical Society

ratio and the number of active sites. On this aspect, recently, catalysts with the size of a cluster or supported single atoms have gained a lot of interest among researchers as ideal catalytic systems.19−22 Second, one can use combination of noble metals with earth-abundant transition metals to achieve highly active composite metallic catalysts. The synergistic effect between different metal atoms has been found in most multimetallic nanocatalysts, and even a small amount of doping could markedly increase the activity and selectivity of catalytic reactions.23−28 In the current study, silica-coated Pt decorated CoCu bimetallic NPs (Pt-CoCu@SiO2) were prepared by a simple spontaneous displacement reaction,29 which exhibited an impressive activity for the catalytic hydrogen production from hydrolysis of AB. In a typical procedure, silica-coated CoCu bimetallic NPs (CoCu@SiO2) were primarily prepared by a modified co-reduction method from reverse microemulsion. After dispersion of CoCu@SiO2 in the aqueous solution of H2PtCl6, Co and Cu atoms on the surface of CoCu bimetallic NPs were partially displaced by Pt4+ ions due to the different reduction potentials of the respective ions, and Pt-CoCu@SiO2 species were generated.16 Pt-CoCu@SiO2 with different Pt loadings can be obtained by changing the concentration of H2PtCl6 solution. The results of catalytic experiments showed Received: October 8, 2016 Revised: December 8, 2016 Published: December 13, 2016 1675

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ACS Sustainable Chemistry & Engineering Scheme 1. Illustration of the Formation Process of CoCu@SiO2

All samples were stored in pure N2 for further characterization and application. Synthesis of Pt-CoCu@SiO2. Pt decorated CoCu bimetallic NPs (Pt-CoCu@SiO2) were synthesized by a spontaneous displacement reaction.29 Typically, 300 mg of CoCu@SiO2 was added into a 250 mL one-neck round-bottom flask containing 54 mL of deionized water; the mixture was heated to 50 °C by water bath under the stirring of 500 r/min for 30 min. Then, 6.0 mL of aqueous solution of H2PtCl6 was added dropwise to the above mixture and maintained at this temperature for another 5 h at the same rate of stirring. The product was collected by centrifugation at 6000 rpm for 5 min. The precipitate was washed with 20 mL of deionized water once and then centrifuged at 6000 rpm for another 5 min; the precipitate was collected and dried at 100 °C for 10 h. Pt-CoCu@SiO2 with Pt loading of 0.43, 0.75, and 1.38 wt % was prepared by changing the concentration of H2PtCl6 aqueous solution from 0.125, to 0.25, to 0.5 mM, respectively (the content of Pt in catalysts was analyzed by ICP-OES after dissolving 20 mg of it with 1 mL of hydrofluoric acid and 1 mL of aqua regia, then diluting the solution to 5 mL). Characterizations. TEM images were captured by a Tecnai G2 20 S-Twin transmission electron microscopy (TEM) at an accelerating voltage of 200 kV. Scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopic (EDS) data were recorded with a JEM-2100F instrument operating at 200 kV. The samples were prepared by dropping an ethanol/water (3:1) dispersion of samples onto carbon-coated copper TEM grids. The XRD patterns were recorded on a Rigaku DMAX IIIVC X-ray diffractometer with Cu Ka (0.1542 nm) radiation scanning from 10° to 80° (2θ) at the rate of 6°/min. X-ray photoelectron spectroscopy (XPS) was acquired by Thermo VG ESCALAB 250 with an Al Kα X-ray source operating at 150 W (15 kV). The binding energies were calibrated using the C 1s peak at 284.6 eV, and the software XPS PEAK 4.1 was used for curve fitting. UV−vis absorbance spectra were obtained by HP 8453. The content of Pt in Pt-CoCu@SiO2 and concentration of metal ions in the supernatant after displacement reaction were determined by the inductively coupled plasma optical emission spectroscopy (Optima 2000 DV, PerkinElmer, ICP-OES). Specific surface area measurement and porosity analysis were characterized using a Quantachrome Autosorb-1-MP surface area and pore size analyzer. Catalytic Hydrolysis of AB. The procedure for catalytic hydrolysis of AB was performed as reported30,31 elsewhere. Generally, the catalyst (80 mg) and distilled water (5.0 mL) were mixed in a twoneck round-bottom flask (50 mL), which was placed in a water bath at 30 °C under ambient atmosphere. A gas buret filled with water was connected to the reaction flask to measure the volume of hydrogen.

an obvious activity enhancement of Pt when supported on CoCu bimetal. The highest turnover frequency (TOF) value was 272.8 molH2 molPt−1 min−1 at the hydrolysis temperature of 30 °C. The improved performance of Pt-CoCu@SiO2 can be attributed to the isolated Pt atoms on the surface of CoCu bimetallic NPs and the synergistic effect between Pt and Co, Cu atoms.

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EXPERIMENTAL SECTION

Materials. Polyoxyethylene (20) cetyl ether (Brij-58) was purchased from Acros. Tetraethyl orthosilicate (TEOS), ammonium hydroxide (NH3·H2O, 25%−28%), cyclohexane, isopropanol (IPA), and hexachloroplatinic acid hexahydrate (H2PtCl6·6H2O) were purchased from Sinopharm Chemical Reagent Co., Ltd. (SCRC). Ammonia-borane complex (90%) was purchased from Aladdin. Cobalt chloride hexahydrate (CoCl2·6H2O) was purchased from Shenyang Reagent Factory. Copper nitrate trihydrate (Cu(NO3)2·3H2O) was purchased from Shantou Xilong Chemical Factory Guangdong. All chemicals were of analytical grade and used without further purification. Deionized water (18.2 MΩ) was used in all experimental processes as needed. Synthesis of CoCu@SiO2 (Co Loading of 3.4 wt %, Cu Loading of 4.0 wt %). In a typical procedure, 3.36 g of Brij-58 (3 mmol) was added to a 50 mL two-neck round-bottom flask containing 15 mL of cyclohexane. The mixture was heated to 50 °C by water bath and turned transparent after stirring for 30 min; then, 0.20 mL of Cu(NO3)2·3H2O and CoCl2·6H2O aqueous solutions (2.0 M) were, respectively, added dropwise to the transparent microemulsion. Due to the addition of a polar phase, the microemulsion turned cloudy, but after 30 min of further stirring, the microemulsion turned transparent again; then, 1.0 mL of ammonium hydroxide was added dropwise. After the reaction mixture stirred for another 30 min, 2.0 g of TEOS was added to the system. To ensure the complete hydrolysis of TEOS, another 2 h of stirring under the temperature of 50 °C was needed, after which 15 mL of IPA was added to demulsify the microemulsion. The mixture was collected and centrifuged at 6000 rpm for 10 min. The precipitate was washed with 30 mL of IPA once and then centrifuged at 6000 rpm for another 10 min; the precipitate was collected and dried at 100 °C for 10 h, and then calcined at 500 °C for 2 h under an air stream with a heating rate of 2.5 °C/min. Finally, the products were collected and reduced at the temperature of 350 °C for 4 h with the heating rate of 2.5 °C/min under H2 (5% H2, 95% N2) with the flow rate of 60 mL/min. For comparison, Cu@SiO2 and Co@SiO2 were also prepared by the same procedure. 1676

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Figure 1. (A) TEM image and (B) HAADF-STEM image of CoCu@SiO2. (C) Particle size distribution of CoCu@SiO2. (D) Particle size distribution of CoCu bimetallic NPs.

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The reaction started when 5 mL of aqueous AB solution (0.75 mmol) was injected into the mixture using a syringe. The volume of the evolved hydrogen gas was monitored by recording the displacement of water in the gas buret. The reaction was completed when there was no more gas generation. The same procedure was also conducted at 20, 30, 40, and 50 °C to determine the Activation Energy (Ea) value for this reaction system. The reaction kinetics were studied under the same catalytic procedure by changing the concentrations of Pt from 0.088, to 0.176, to 0.256, to 0.353 mM, and the concentrations of AB from 36, to 72, to 108, to 144 mM, respectively. Durability Test. The procedure for the durability test was similar as described above; typically, 80 mg of Pt-CoCu@SiO2 with the Pt loading of 0.43 wt % was added to a two-neck round-bottom flask (50 mL) containing 5 mL of distilled water under the stirring of 500 r/min and temperature of 30 °C. The reaction started when 5 mL aqueous AB solution (0.75 mmol) was injected into the mixture. The volume of the evolved hydrogen gas was monitored by recording the displacement of water in the gas buret. First run was completed when no more gas was generated. Then, the catalyst was separated by centrifugation at 6000 rpm for 5 min and washed with 10 mL of distilled water. After the centrifugation at 6000 rpm for another 5 min, the precipitate was collected for the next run. Calculation Method. The turnover frequency (TOF) value calculated here was a TOF value based on the number of Pt atoms added in the reaction system.12 The equation for TOF calculation is given below:

TOF =

RESULTS AND DISCUSSION CoCu bimetallic NPs encapsulated in silica (CoCu@SiO2) were prepared by a modified co-reduction method using a reverse microemulsion synthesis. A simplified illustration of the synthesis of CoCu@SiO2 is depicted in Scheme 1. The reverse microemulsion was adopted because its micelles can work as nanoreactors during the synthesis. These nanoreactors ensure better distribution of metal ions and provide an accurate elemental control during the preparation of multimetallic NPs. During the synthesis, the CoCu NPs were formed by the aggregation of the metal clusters under H2 reduction at high temperature. This aggregation was achieved due to the reduced melting points and increased surface energy of the metals in the cluster size regime. The morphology of the as-prepared CoCu@SiO2 was analyzed by TEM. Figure 1A shows a representative TEM image of CoCu@SiO2 (large scale TEM image is shown in Figure S2, Supporting Information). It can be seen in Figure 1A that CoCu@SiO 2 NPs are monodispersed with good morphology. The average size of CoCu@SiO2 was 27.8 ± 2.0 nm (Figure 1C) which was calculated from 100 random particles; meanwhile, a single core with an average size of 4.8 ± 0.6 nm (Figure 1D) can also be observed. High-angle annular dark-field STEM (HAADF-STEM) was conducted to study the detailed structure of CoCu@SiO2; the image is shown in Figure 1B. Besides the single CoCu core, a small amount of metal clusters can also be observed. The formation of cores can be attributed to the aggregation of ultrasmall metal clusters. The existence of randomly distributed metal clusters may have resulted due to the in situ reduction of bigger CoO/CuO clusters having melting points higher than the temperature during reduction. The EDS point analysis was used to confirm

VH2 22.4VsC Ptt

The abbreviations used here are as follows: VH2 was the total volume of hydrogen generated, Vs was the volume of solution (10 mL in this report), CPt was the concentration of Pt in the reaction mixture, and t was the reaction time. The unit of TOF used in this study was molH2 molPt−1 min−1. 1677

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Figure 2. HAADF-STEM image of as-synthesized CoCu@SiO2 and EDS point analysis spectrum of the selected area.

the composition of CoCu bimetallic NPs (Figure 2). Both the signals of Cu and Co elements can be detected in the bright spot, indicating the formation of CoCu bimetallic NPs. The EDS elemental mapping of CoCu@SiO2 further confirmed the existence of Cu, Co, Si, and O which is presented in Figure S3 (Supporting Information). To further confirm the core−shell structure of CoCu@SiO2 and the formation of CoCu bimetallic NPs, the surface element compositions and the corresponding chemical status of CoCu@SiO2 were analyzed by XPS and shown in Figure S4 (Supporting Information). Figure S4A (Supporting Information) shows the wide scan spectrum of CoCu@SiO2 in which elements Cu, Co, O, and Si can all be detected. The contents of elements Cu and Co were 1.0 and 2.2 wt % by the XPS analysis which were much lower than the ICP results. The reason for this phenomenon lies in the limit of detection by XPS which can only analyze the elements on the surface of samples with the thickness of 5−10 nm. As the average size of CoCu@SiO2 particles has been measured as 27.8 nm, it is reasonable to declare that most Cu and Co elements were in the core of CoCu@SiO2. Figure S4B,C (Supporting Information) shows the high-resolution spectra for the elements Cu and Co; the binding energies of 933.4 and 953.2 eV can be attributed to Cu 2p3/2 and Cu 2p1/2, respectively, which match the metallic Cu0. The binding energies of 782.6 and 797.8 eV can be attributed to Co 2p3/2 and Co 2p1/2, corresponding to Co2+ which also means the element Co can more easily be oxidized than Cu on the surface of CoCu bimetallic NPs. The XRD patterns of Cu@ SiO2, Co@SiO2, and CoCu@SiO2 are shown in Figure S5 (Supporting Information). The broad peaks around 25° for all three samples can be attributed to the amorphous silica. Both Cu@SiO2 and Co@SiO2 show peaks consistent with the standard cards (JCPDS 44-0962 for Co and JCPDS 03-1015 for Cu), indicating a face-centered cubic structure. Due to the small size and poor crystallinity of CoCu bimetallic NPs, the XRD pattern of CoCu@SiO2 appeared as a weak but detectable diffraction peak around 43°. Actually, the poor crystallinity of CoCu bimetallic NPs proved advantageous for the catalytic reactions, and the details will be discussed in the latter part of this report. It has been reported that supported Pt NPs show high activity for the hydrogen generation from hydrolysis of AB.15,32 CoCu bimetallic NPs also exhibit efficient catalytic activity in the hydrolysis of AB for hydrogen generation.33,34 Thus, it is reasonable to design an efficient catalyst combining the Pt atoms with CoCu bimetallic NPs. This approach could not only reduce the amount of noble metal; i.e., with Pt being used, it would also provide an insight into the synergistic effect between Pt and Co, Cu atoms. In this work, a minor amount of Pt decorated CoCu bimetallic NPs (Pt-CoCu@SiO2) were synthesized by a simple spontaneous displacement reaction.29

The possible synthesis process is shown in Scheme 2; after the addition of H2PtCl6 aqueous solution into the suspension of Scheme 2. Simulated Formation Process of Pt-CoCu@SiO2 by Spontaneous Displacement Reaction

CoCu@SiO2, partial Cu and Co atoms on the surface of CoCu bimetallic NPs can be replaced by the Pt atoms. This can be explained by considering the reduction potentials of the respective ions [E° (Cu2+/Cu) = 0.339 V, E° (Co2+/Co) = −0.282 V vs E° ([PtCl6]2−/[PtCl4]2−) = 0.68 V, E° ([PtCl4]2−/ Pt) = 0.847 V]. The percentage of exposed surface atoms of CoCu bimetallic NPs is significant in determining the number of metal atoms that can be replaced. Calculations of the number of exposed surface atoms were performed according to the Mackay model35,36 which is based on the assembly of n shells around a central atom in a group of N atoms. Mackay model is given by N = (10/3)n3 + 5n2 + (11/3)n + 1

The number of atoms in the nth shell is given by Nn = 10n2 + 2

Suppose the average diameter of CoCu bimetallic NPs is 4.8 nm, and the calculated number of shells is nearly 9 (the atomic radius of Co and Co are 0.125 and 0.128 nm, respectively). The calculated percentage of the exposed surface atoms is 28.3%. In this study, the atomic ratios of Pt:(Co + Cu) = 1%, 2%, 4% corresponding to the Pt loading of 0.43, 0.75, and 1.38 wt % were chosen for the design of Pt-CoCu@SiO2. As the atomic ratio of Pt is much lower than the calculated percentage of exposed surface atoms (28.3%), we suppose all Pt atoms were on the surface of CoCu bimetallic NPs. Figure 3A shows the TEM images of Pt-CoCu@SiO2 (Pt loading of 0.43 wt %); it is difficult to directly observe the existence of Pt atoms on the surface of CoCu bimetallic NPs by TEM, and no clear difference can be observed from both the morphology and the size of CoCu bimetallic NPs due to the very small amount of Pt loading. To confirm the reaction between Pt4+ ions and CoCu bimetallic NPs, the mixture was centrifuged after the displacement reaction; the supernatant 1678

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Figure 3. (A) TEM image of Pt-CoCu@SiO2, (B) XPS spectra of element Pt in Pt-CoCu@SiO2, (C) EDS analysis of Pt-CoCu@SiO2, (D) HAADFSTEM image of Pt-CoCu@SiO2 and the corresponding element mapping for Si, O, Cu, Co, and Pt.

Table 1. ICP-OES Analysis of Supernatant after Displacement Reactiona ICP content sample

initial conc of Pt4+ (mol/L)

Co (mg/L)

Cu (mg/L)

conc of (Co2+ + Cu2+) in supernatant (mol/L)

percentage of Pt4+ reducedb (%)

Pt-CoCu@SiO2 (0.43 wt %) Pt-CoCu@SiO2 (0.75 wt %) Pt-CoCu@SiO2 (1.38 wt %)

1.25 × 10−4 2.5 × 10−4 5.0 × 10−4

9.770 10.65 8.085

3.359 12.94 36.08

2.186 × 10−4 3.844 × 10−4 7.050 × 10−4

87.45 76.87 70.50

a

The initial concentrations of Pt4+ were the concentrations of H2PtCl6 added to the system for displacement reactions. bAs the reduction of one Pt4+ ion can induce the oxidation of two Cu atoms or Co atoms, thus percentage of Pt4 +reduced =

conc of (Co2 + + Cu 2 +) in supernatant × 100 2 × initial concs of Pt4 +

was collected, and the concentrations of Co2+ and Cu2+ in the supernatant were analyzed by ICP-OES. The results are shown in Table 1. As can be seen, the concentration of Co2+ and Cu2+ ions in the supernatant increased accordingly with the increasing loading amount of Pt in catalyst which indicated the occurrence of the displacement reaction. Meanwhile, for all

samples, more than 70% Pt4+ ions can be reduced on the basis of the calculations (according to the equation in Table 1, footnote b). The UV−vis absorbance analysis was also used to determine the concentration of ions in the supernatant (Figure S6, Supporting Information). For comparison, the absorbance spectra of H2PtCl6, Cu(NO3)2, and CoCl2 aqueous solutions 1679

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ACS Sustainable Chemistry & Engineering with the concentration of 0.05 mM were also recorded. After 5 h of displacement reaction, the peaks of Pt4+ at 198 and 261 nm decreased, which means that, with the reduction of Pt4+, the peak of Cu2+ at 200 nm appeared in the supernatant indicating the replacement between Pt atoms and Co, Cu atoms (Co2+ showed no absorbance above 190 nm). Figure S7A (Supporting Information) shows the UV−vis absorbance spectra of the Cu(NO3)2 aqueous solution with different concentrations, and Figure S7B (Supporting Information) shows the fitted line for absorbance−concentration relation of Cu2+ at 200 nm. The absorbance intensity of Cu2+ showed a good linear relation with its concentration which also provides a simple way to analyze the concentration of Cu2+ and Co2+ ions in supernatant. Figure S7C (Supporting Information) shows the UV−vis absorbance spectra of supernatants after the displacement reaction by H2PtCl6 aqueous solutions with different concentration; the intensity of the characteristic peak of Cu2+ at 200 nm increased with the increasing concentration of H2PtCl6 also indicating the occurrence of the displacement reaction. The XPS analysis (Figure 3B) proved the existence of Pt element in the final product Pt-CoCu@SiO2 with the Pt content of 0.49 wt % which is slightly higher than the ICP result (0.43 wt %). The binding energy of 69.39, 73.15, and 75.83 eV can be attributed to Pt 4f5/2 of Pt0, Pt2+, and Pt4+, respectively. The XPS results also showed that Pt atoms on the surface of CoCu bimetallic NPs can be partially oxidized.37 The XPS analysis of elements Co and Cu in Pt-CoCu@SiO2 was carried out, and the corresponding spectra are shown in Figure S8 (Supporting Information). The binding energies of 933.4 and 953.2 eV can be attributed to Cu 2p3/2 and Cu 2p1/2 of metallic Cu which were similar to the binding energy of Cu in CoCu@SiO2 (Figure S4, Supporting Information). The XRD patterns of CoCu@SiO2 and Pt-CoCu@SiO2 are shown in Figure S9 (Supporting Information); the diffraction signal for Pt is imperceptible due to the low loading of Pt which also implies that no isolated Pt particles were formed during this replacement reaction. Figure 3C shows the EDS analysis result of Pt-CoCu@SiO2; the signals for elements Co, Cu, and Pt can all be observed. Figure 3D shows the HAADF-STEM image and element mapping of Pt-CoCu@SiO2. The bright spheres represent the generated Pt-CoCu particles. The signals for element Cu, Co, and Pt can all be clearly observed through the whole sample region in the mapping results indicating the formation of Pt-CoCu particles. Prior to the catalytic studies, N2 sorption measurements were carried out to investigate the surface area and pore size distribution of the prepared catalysts. The results are shown in Figure S10 and Table S1 (Supporting Information). The surface area of CoCu@SiO2 was determined to be 91.5 m2/g; the sorption isotherms are typical type IV according to the IUPAC classification and showed a clear increase at low pressure, indicating the micro- or mesoporous feature. As no obvious porous structure can be observed by the TEM, the mesoporous nature can be assigned to interparticle voids. Meanwhile, the pore size was calculated to be 0.5 and 1.0 nm which is similar to results from our early report.38 The catalytic activity of the as-prepared Co@SiO2, Cu@SiO2, and CoCu@ SiO2 was evaluated. Figure 4 shows the time courses for hydrogen production from hydrolysis of AB. Co@SiO2 showed low activity with only 6% of AB decomposed in 180 min, which was consistent with the reported results.23,31 Cu@SiO2 showed higher activity than Co@SiO2, with 22% of AB decomposing in 240 min. In a comparison with Co@SiO2 and Cu@SiO2, the

Figure 4. Time course for hydrogen production from AB using Co@ SiO2, Cu@SiO2, and CoCu@SiO2.

activity of CoCu@SiO2 was much enhanced with 80% of AB decomposed in 210 min which means the synergistic effect between Co and Cu atoms was observed in our catalyst, even though the combination of Co and Cu showed improved performance but still cannot meet the complete decomposition of AB for practical usage. The effect of different supports on the catalytic activity of Pt was evaluated, and the time courses for hydrogen production from hydrolysis of AB are shown in Figure 5A. It turns out that Co@SiO2 did not show any enhanced activity after the introduction of Pt, with a TOF value of 0.36. Different from Co@SiO2, Pt decorated Cu@SiO2 showed much enhanced activity compared to that of Cu@SiO2; 79.6% of AB decomposed in 30 min with a TOF value of 29.01. What is more interesting is that Pt decorated CoCu@SiO2 showed an unexpected high activity; 100% of AB decomposed in only 8 min with a TOF value of 136.7. The reason for this activity difference can be attributed to the modified electronic structure of Pt atoms by the interactions with Co, Cu, and CoCu atoms. The formation of CoCu bimetallic NPs as support of Pt atoms can significantly enhance its catalytic activity. Further, to confirm that the activities of catalysts are not coming from the absorbed Pt4+ ions, Pt decorated pure SiO2 was also prepared by the same method as the control sample which showed no activity. The catalytic activity of Pt-CoCu@SiO2 with different Pt loadings was also tested. The total content of Pt was maintained the same for all catalytic reactions. Figure 5B shows the time courses for hydrogen production; the most active catalyst was Pt-CoCu@SiO2 with the Pt loading of 0.43 wt %, and the TOF value reached 272.8, which is an impressive value in comparison with other Pt based catalysts recently reported for this reaction (Table 2). The activity decreased with the increase of Pt loading, (TOF value) 181.9 and 136.7 for PtCoCu@SiO2 with Pt loading of 0.75 and 1.38 wt %, respectively. To explain this phenomenon, the simulated distribution of Pt atoms on CoCu bimetallic NPs is shown in Figure S11 (Supporting Information). As the loading of Pt increased from 0.43, to 0.75, to 1.38 wt %, the isolated single Pt atoms decreased, and the synergistic effect between Co, Cu atoms and Pt atoms was weakened. Generally, the crystallinity of metallic catalysts shows a great influence on its catalytic activity. In this study, by changing the reducing temperature, CoCu bimetallic NPs with different crystallinity were prepared and used as starting materials for the 1680

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Figure 5. Time courses for hydrogen production from hydrolysis of AB using (A) Pt decorated SiO2, Co@SiO2, Cu@SiO2, and CoCu@SiO2; (B) Pt-CoCu@SiO2 with different Pt loading; (C) CoCu@SiO2 prepared under different reducing temperature with the Pt loading of 0.43 wt %; and (D) the corresponding TOF values calculated from the results of parts A−C.

Table 2. Comparison of TOF Values and Activation Energy with the Recently Reported Heterogeneous Catalysts for Dehydrogenation of Ammonia-Boranea

a

catalyst

T/°C

TOF/molH2 molPt−1 min−1

commercial Pt/C Pt25@TiO2 Pt NPs/MIL-101 Pt/CNT Pt@SiO2 PtNi@SiO2 Pt@SiO2 Ru@SiO2 Ni@SiO2 Co@SiO2 Cu@SiO2 Pt0.65Ni0.35 PdPt cNPs PdPt sNPs Pt-CoCu@SiO2

25 25 25 30 30 30 25 25 25 25 25 30 25 25 30

83.3 311 414 464 1.79 20.7 158.6 200 18.5 12 3.2 44.3 50.02 22.51 272.8

activation energy/kJ mol−1

41 54.76 53.9 38.2

36.0 39.0 21.8 57.3 51.01

ref 9 13 14 15 39 39 40 41 42 43 44 45 46 46 this work

The TOF values were taken directly from the related articles or calculated on the basis of the data given.

the loss of defects on the surface of CoCu bimetallic NPs which work as the active sites. To study the reaction kinetics of AB hydrolysis catalyzed by Pt-CoCu@SiO2, the hydrogen generated versus time in the presence of different concentrations of Pt and AB were tested. Figure 6A shows the time courses for hydrogen production from hydrolysis of AB using different Pt concentrations. The reaction rates can be determined from the linear portion of each plot. The inset shows the relation between the reaction rates and the Pt concentration in logarithmic scale. It can be seen that the hydrolysis of AB is a first order reaction with respect to the Pt concentrations. With a change in the addition amount of substrate, the influences of AB concentrations on the reaction rate were also evaluated. Figure 6B shows the relation

synthesis of Pt-CoCu@SiO2. Figure S12 (Supporting Information) shows the XRD patterns of CoCu@SiO2 prepared under different reducing temperatures. The crystallinity of CoCu bimetallic NPs increased with the increase in reducing temperature. For the synthesis of Pt-CoCu@SiO2, the Pt loading was maintained as 0.43 wt % for all samples. Figure 5C shows the time courses for hydrogen production from hydrolysis of AB. As the reducing temperature for the preparation of CoCu@SiO2 increased, the TOF values first increased and then decreased. The highest TOF value was achieved to be 272.8 when the reducing temperature was 350 °C. As the temperature was raised to 550 °C, the TOF value decreased to 182.2. The decrease in activity can be ascribed to 1681

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Figure 6. Time courses for hydrogen production from hydrolysis of AB using (A) different catalyst concentrations (30 °C, [AB] = 72 mM, Pt loading of 0.43 wt %) and (B) different AB concentrations (30 °C, [Pt] = 0.176 mM, Pt loading of 0.43 wt %). The inset shows the plot of the hydrogen production rate as a function of the concentrations of Pt and AB, respectively.

Figure 7. (A) Time courses for hydrogen production from hydrolysis of AB using Pt-CoCu@SiO2 (Pt loading of 0.43 wt %) under different hydrolysis temperatures (the inset shows the corresponding TOF values). (B) The Arrhenius plots for the calculation of activation energy (Ea).

between generated hydrogen volumes versus time. The volume of generated hydrogen increased accordingly with the increasing of AB concentrations. The inset of Figure 6B shows the relation between the reaction rate and the AB concentrations. The straight line with a slope of −0.006 indicates a zero order reaction for the hydrolysis of AB with respect to the AB concentration; hence, the reaction rate is independent with the concentration of AB. It is generally believed that temperature has a great influence on the hydrogen generation rate from hydrolysis of AB. Figure 7A shows the time courses for hydrogen production during the catalytic hydrolysis of AB at various temperatures in the range 20−50 °C. The hydrolysis of AB was completed from 495 to 70 s when the temperature was raised from 20 to 50 °C, with the TOF value increasing from 132.3 to 935.5 which is shown in the inset. The activation energy was also calculated to be 51.0 kJ/mol from the corresponding Arrhenius plots, and as shown in Figure 7B, the value is similar to many reports for the same reaction (Table 2).39 The durability of a catalyst plays an essential role for its practical applications. The stability of Pt-CoCu@SiO2 was tested by recycling the catalyst after each hydrolysis reaction. The detailed procedure for recycling is described in the Experimental Section. Figure 8 shows the time courses for hydrogen production from hydrolysis of aqueous AB solution in six runs. The activity of Pt-CoCu@SiO2 only showed a slight decrease, which demonstrated its high durability for AB hydrolysis. The TEM image of Pt-CoCu@SiO2 after the

Figure 8. Durability characterization of Pt-CoCu@SiO2 (Pt loading of 0.43 wt %) in six runs for hydrogen generation from hydrolysis of aqueous AB solution.

recycling test is shown in Figure S13 (Supporting Information); the morphology of the catalyst was maintained, and no obvious aggregation was observed. The ICP result showed that the content of Pt was slightly decreased from 0.43 to 0.39 wt % after recycling; this might be one reason for the deactivation of catalyst. The loss of catalytic materials during the separation of catalyst in each step might be another reason for the decrease of its catalytic activity (the amount of catalyst was decreased from the initial 80 mg to final 74 mg after the sixth run). Finally, the 1682

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good durability of the catalyst can be attributed to the protection of the silica shell from the aggregation of metals during the catalytic reactions and preservation of the active sites.

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CONCLUSIONS In summary, a minor amount of Pt decorated CoCu@SiO2 is successfully synthesized for the efficient catalytic hydrogen evolution from hydrolysis of ammonia-borane. Pt-CoCu@SiO2 prepared at the reducing temperature of 350 °C and Pt loading of 0.43 wt % exhibited the best catalytic activity with TOF value reaching 272.8 molH2 molPt−1 min−1 at 30 °C. The activation energy of the same catalyst for the hydrolysis of AB was calculated to be 51.01 kJ/mol. The remarkable enhancement in the catalytic activity was attributed to the synergistic effect of Pt and Co, Cu atoms. In addition, Pt-CoCu@SiO2 showed high stability for the hydrogen production from AB in six runs. The long durability of the catalyst could be attributed to the protection of the silica shell which prevented the aggregation of metals during the catalytic reactions and preserved the active sites.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02430. TEM images, STEM image, EDS element mapping, XPS spectra, UV−vis absorbance spectra, XRD patterns, nitrogen adsorption−desorption isotherms, pore size distributions, and simulated distribution of Pt atoms on CoCu bimetallic nanoparticles (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yuzhen Ge: 0000-0002-2133-1509 Zameer Hussain Shah: 0000-0002-8249-6403 Rongwen Lu: 0000-0002-2210-3380 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grants 21576044, 21536002), the Science Fund for Creative Research Groups of the National Natural Science Foundation of China (Grant 21421005), the Fundamental Research Foundation of Liaoning Ministry of Education (Grant LZ2015020), the Fundamental Research Funds for the Central Universities (DUT15ZD224), and Dalian University of Technology Innovation Team (DUT2016TB12).

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