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Preparation of bimetallic Cu-Co nanocatalysts on poly (diallyldimethylammonium chloride) functionalized halloysite nanotubes for hydrolytic dehydrogenation of ammonia borane
Applied Surface Science 427 (2018) 106–113
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Preparation of bimetallic Cu-Co nanocatalysts on poly (diallyldimethylammonium chloride) functionalized halloysite nanotubes for hydrolytic dehydrogenation of ammonia borane Yang Liu, Jun Zhang, Huijuan Guan, Yafei Zhao, Jing-He Yang ∗ , Bing Zhang ∗ School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou, 450001, PR China
a r t i c l e
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Article history: Received 7 June 2017 Received in revised form 23 August 2017 Accepted 24 August 2017 Keywords: Halloysite nanotubes Ammonia borane Bimetallic copper-cobalt Hydrolytic dehydrogenation Polyelectrolytes modification
a b s t r a c t In present work, we prepared the bimetallic Cu-Co nanocatalysts on poly (diallyldimethylammonium chloride) functionalized halloysite nanotubes (Cu-Co/PDDA-HNTs) by a deposition-reduction technique at room temperature. The analysis of XRD, SEM, TEM, HAADF-STEM and XPS were employed to systematically investigate the morphology, particle size, structure and surface properties of the nanocomposite. The results reveal that the PDDA coating with thickness of ∼15 nm could be formed on the surface of HNTs, and the existence of PDDA is beneficial to deposit Cu and Co nanoparticles (NPs) with high dispersibility on the surface. While the cost-effective nanocomposite was used for the hydrolytic dehydrogenation of ammonia-borane (NH3 BH3 ), the nanocatalyst showed extraordinary catalytic properties with high total turnover frequency of 30.8 molH2 /(molmetal min), low activation energy of 35.15 kJ mol−1 and high recycling stability (>90% conversion at 10th reuse). These results indicate that the bimetallic Cu-Co nanocatalysts on PDDA functionalized HNTs have particular potential for application in release hydrogen process. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen is considered to be one of the most promising energy carriers for the future owing to its clean burning nature, environmental friendliness, and high energy content. However, the hydrogen storage method that can release hydrogen safely and efficiently under ambient condition still faces many challenges for the future “hydrogen economy” [1,2]. Among the new hydrogen storage materials, NH3 BH3 (AB) appears to be the most promising material because of its low molecular weight (30.9 g mol−1 ), nontoxicity, and high hydrogen density (19.6 wt%), which is greater than the target value of U.S. Department of Energy (5.5 wt% H2 ) [3–5]. More importantly, with an appropriate catalyst, catalytic hydrolysis of AB can release 3 mol H2 per mol AB at room temperature [6]. So far, a wide range of catalysts based mostly on transition-metal NPs have been developed for the hydrolytic dehydrogenation of AB including noble metals and non-noble metals such as Pt, Ru, Pd, Co, Cu and Ni [6–14]. Bimetallic nanocatalysts usually have higher cat-
∗ Corresponding author. E-mail addresses: [email protected] (J.-H. Yang), [email protected] (B. Zhang). http://dx.doi.org/10.1016/j.apsusc.2017.08.171 0169-4332/© 2017 Elsevier B.V. All rights reserved.
alytic performance than their monometallic counterparts, owing to the synergistic geometric and electronic effects of the bimetallic NPs, such as Co-Pd, Ni-Mo and Cu-Ni, Cu-Co [15–20]. To be active in the catalytic reaction, metal catalysts are required to have an ultrafine and uniform size, which cause the increase in the number of active sites. However, these metallic NPs often suffer from insufficient stability/durability owing to agglomeration [21]. Recently, solid support materials have received considerable attention, which could effectively restrain the agglomeration of metal NPs and enhance the catalytic activities. Meanwhile, the supports could limit the migration of the active components and thus enhance the long-term stability [22]. Halloysite nanotubes [Al2 Si2 O5 (OH)4 ·nH2 O, HNTs] are cheap and have an abundant natural source in many countries, such as Australia, New Zealand, China, Guyana, Mexico, and Brazil. HNTs consist of a 1:1 layered aluminosilicate mineral that has a predominantly hollow tubular structure, which offer potential applications as supports for catalytic composites with distinctly improved stability and mechanical property [23–25]. However, natural HNTs exhibit a low adhesion for bonding metal NPs because of the absence of chemical conjunction, which leads to easy leaching of the metal particles from the surface during reactions [26]. To solve this problem, one of the methods is the effective surface functionalization of HNTs, which makes it possible to improve adhesive
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capacity of metal NPs on their surface. PDDA, a water soluble quaternary ammonium polyelectrolyte, is renowned as an environmentally friendly and low cost polyelectrolyte [27]. Previous studies reported the use of PDDA as a reducing agent and a stabilizer for fabricating nanostructured metals. In addition, PDDA also can be utilized to prepare composite materials via surface modification of carbon-based materials (e.g. graphene and carbon nanotubes) for subsequent NP deposition [28–30]. However, up to now, functionalized HNT with PDDA as the substrate for the catalytic field has been rarely reported. In this work, we immobilized the bimetallic Cu-Co NPs on HNT decorated with PDDA by a facile deposition-reduction method, and employed it as catalyst for hydrogen generation from the hydrolysis of AB at room temperature. We found that the synthesized Cu-Co/PDDA-HNT nanocomposites exhibit higher catalytic activity, in comparison to their monometallic counterparts. Especially, Cu0.5 Co0.5 /PDDA-HNT shows the best catalytic activities among the Cux Co1−x /PDDA-HNT system, a high turnover frequency, a low activation energy and a good recycling stability in the hydrolytic dehydrogenation of AB at room temperature. 2. Experimental 2.1. Chemicals All chemical reagents were obtained from commercial suppliers and used without further purification. Halloysite nanotubes (HNTs) were refined from clay minerals in Henan province, China. Poly (diallyldimethylammonium chloride) (PDDA, Mw = 100,000–200,000) in 20% aqueous solution, cobalt (II) sulfate heptahydrate (CoSO4 ·7H2 O, 99%) and ammonia borane complex (NH3 BH3 , AB, 97%) were purchased from Aldrich, copper (II) chloride dehydrate (CuCl2 ·2H2 O, 99%) was purchased from Tianjin Fengchuan Chemical Reagent Co. Ltd. China., sodium borohydride (NaBH4 , 97%) was purchased from Sinopharm Chemical Reagent Co. Ltd. China. All other regents were of analytical reagent grade. The water was used deionized water.
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followed the same process. The metal content of the product (Cu + Co)/(Cu + Co + PDDA-HNT) was kept at about 10 wt%. 2.4. Determination of the catalytic activity of Cu-Co/PDDA-HNT nanocomposites in the hydrolytic dehydrogenation of AB The hydrolysis equation of AB is briefly presented as follows: NH3 BH3 + 2H2 O → NH4 + + BO2 − + 3H2 ↑ The volume of released gas was monitored using the gas burette by water displacement during the AB catalyzed by the catalysts. In a typical experiment, a three necked reaction flask (25.0 mL) containing the as-synthesized nanocomposites suspension (4.0 mL) was placed on a magnetic stirrer and thermostated to 298 K by using a constant temperature bath. The left and middle necks were sealed with the glass stoppers, the right neck was sealed with the glass stopper which was connected to the gas-collection tube. After the suspension was stirred 15 min, 1.0 mL aqueous solution of 32 mg AB was added into the reaction flask by the middle neck then kept in the glass stopper. The flask was well sealed with vaseline. The reaction time was recorded from the appearance of the first bubble. The reaction was finished when there was no more gas evolved. Finally, the catalytic activity of Cu-Co/PDDA-HNT samples in the hydrolysis of AB was evaluated by the turnover frequency (TOF) value. According to the references [32,33], the TOF numbers are calculated by a typical formula: TOF = nH2 /(nmetal × t), in which nH2 is the mole number of generated H2 , while nmetal is the total mole number of metal (Cu and Co), t is the completed reaction time (min). The catalytic reactions were also carried out at different temperatures ranging from 298 to 313 K under an ambient atmosphere to get the activation energy (Ea) of the AB hydrolysis catalyzed by CuCo/PDDA-HNT nanocomposites. The Ea is calculated according to the following Arrhenius equation [11]: lnk = lnA − Ea/RT
2.2. Synthesis of PDDA-HNTs PDDA/HNTs were prepared according to the reported method with a slight modification [31]. Briefly, 0.5 g of HNTs was dispersed into 50.0 mL of a 2.5 wt% PDDA aqueous solution containing 0.5 M NaCl and ultrasonicated for 1 h to obtain a homogeneous white suspension and then rapid magnetic stirring for 24 h was used to ensure the presence of well dispersed PDDA-HNTs. The dispersion was centrifuged and washed with water several times to remove redundant PDDA and then the collected precipitate was dried under vacuum at room temperature to get the PDDA-HNTs.
2.5. Reusability performance of Cu0.5 Co0.5 /PDDA-HNT nanocomposite in the hydrolytic dehydrogenation of AB After the first run of hydrolytic dehydrogenation of AB solution (0.2 M, 5 mL) starting with Cu0.5 Co0.5 /PDDA-HNT at 298 K, the catalyst was isolated from reaction solution by centrifugation and washed with excess water, then dried in vacuum oven at room temperature. The dried nanocomposite was used again in the hydrolytic dehydrogenation of AB solution (0.2 M, 5 mL) at 298 K. Such a recycle test of the composites for the hydrolysis of AB was carried out for 10 runs under an ambient atmosphere at room temperature.
2.3. Preparation of Cu-Co/PDDA-HNT nanocomposites 2.6. Characterization Cu0.5 Co0.5 /PDDA-HNT nanocomposites were synthesized as follows: 50.0 mg of the prepared PDDA/HNTs was dispersed in 15.0 mL aqueous solution, to which 5.0 mL of aqueous solution containing 7.8 mg CuCl2 ·2H2 O and 12.8 mg CoSO4 ·7H2 O. After stirring at room temperature for 2 h, 5.0 mL aqueous solution of 55.6 mg NaBH4 was added by drop to the above mixture and the resulting solution was stirred for half an hour under air at room temperature. After centrifugation (8000 rpm, 5 min), copious washing with water, filtration, and drying in vacuum oven at room temperature. Cu0.5 Co0.5 /PDDA-HNT nanocomposite was obtained as green powders and stored in vacuum oven at room temperature. The synthesis of other Cux Co1-x /PDDA-HNT nanocomposites with different Cu molar content (x = 0, 0.1, 0.3, 0.7, 0.9, 1.0)
The surface morphology of the nanocatalysts were characterized by scanning electron microscopy (SEM, Hitachi S2400). The morphology and crystal structure of the samples were observed by high resolution transmission electron microscopy (HRTEM) and high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) on a FEI Talosf200S operating at an acceleration voltage of 200 kV. The crystal structures of the products were characterized by X-ray diffraction (XRD) on a D8ADVANCE in the range of 5–75◦ . The chemical compositions of the catalysts were analyzed by using ELAN 9000 inductively coupled plasma atomic mass spectrometer (ICP-MS). X-ray photoelectron spectroscopy (XPS) measurement was performed with
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Y. Liu et al. / Applied Surface Science 427 (2018) 106–113 Table 1 ICP-MS analysis of Cux Co1-x /PDDA-HNT nanocomposites.
Fig. 1. The X-ray diffraction (XRD) patterns of (a) HNT, (b) PDDA-HNT, (c) Cu0.5 Co0.5 /PDDA-HNT.
a Thermo Scientific-ESCALAB 250XI multifunctional imaging electron spectrometer to study the surface properties of the catalysts. 3. Results and discussion 3.1. Characterization of Cu-Co/PDDA-HNT nanocomposite Fig. 1 depicts XRD patterns of HNT, PDDA-HNT, and Cu0.5 Co0.5 /PDDA-HNT materials. In the XRD pattern of HNTs, all of the observed peaks can be indexed to the characteristic peaks of halloysite (JCPDS No.09-0453) [34]. The peak appearing at 2 of 11.79◦ , 20.07◦ , 24.50◦ , 35.02◦ correspond to (001), (100), (002), (110) crystal planes respectively are clearly observed in Fig. 1a. The PDDA-HNT and HNT show similar peaks in Fig. 1b [35]. Apart from the reflection peaks of halloysite itself, two additional intense peaks are observed for Cu0.5 Co0.5 /PDDA-HNT nanoconposite at 43.3◦ , 36.5◦ , which can be assigned to Cu [111] (JCPDS No.04-0836) and Cu2 O [111] (JCPDS No.05-0667). The main reason for the observation of cuprous oxide phases is the high oxophilicity of copper (0), which is due to the partly surface oxidation of Cu NPs and could not be avoided [3]. However, the Co NPs are not observed from XRD in the sample, which may be due to the fact that the cobalt NPs are too small or amorphous [19,36]. XRD patterns of Cux Co1-x /PDDA-HNT (x = 0, 0.1, 0.3, 0.7, 0.9, 1.0) nanocomposites are shown in Fig. S1. SEM and TEM images of HNTs and Cu0.5 Co0.5 /PDDA-HNTs are presented in Fig. 2a–d. The structure and morphology of the natural HNTs in Fig. 2a and b. It can be seen that natural HNTs mainly consist of hollow nanotubes in various sizes. The HNTs have lengths ranging from 0.5 to 1 m, diameters in the range of 40–60 nm. Fig. 2c and d exhibits morphology and structure of Cu0.5 Co0.5 /PDDA-HNTs. The distinct morphology shows that PDDA layer has successfully coated on surface of HNTs in Fig. S2 and the Cu0.5 Co0.5 /PDDA-HNTs nanocomposites have a slight agglomeration in Fig. 2c. Fig. 2d shows the tubular structure disappears, PDDA layer may diffuse into the inner surface of HNTs. We also notice that highly dispersed ultrafine NPs immobilized on the outer layer. The HAADF-STEM images of Cu0.5 Co0.5 /PDDA-HNTs in Fig. S3 also show the ultrafine NPs immobilized on the surface of the PDDAHNTs and the PDDA coating with thickness of ∼15 nm could be found from the surface of HNTs. Moreover, the size distribution of NPs for Cu0.5 Co0.5 /PDDA-HNT is estimated by Nano Measurer software, as shown in inset of Fig. 2d. The mean metal NPs size is found to be ∼2.2 nm. Additionally, the structural characterization of Cu and Co NPs decorating the PDDA-HNT in further detail is achieved using HRTEM in Fig. 2e. The lattice fringes with spacing of 0.209 nm and 0.200 nm are clearly visible in NPs on PDDA-
Catalysts
Cu loading wt%
Co1.0 /PDDA-HNT Cu0.1 Co0.9 /PDDA-HNT Cu0.3 Co0.7 /PDDA-HNT Cu0.5 Co0.5 /PDDA-HNT Cu0.7 Co0.3 /PDDA-HNT Cu0.9 Co0.1 /PDDA-HNT Cu1.0 /PDDA-HNT
1.0 2.9 4.4 7.0 8.7 9.7
Co loading wt% 9.7 8.3 6.3 4.6 2.7 0.7
HNT, which are consistent with the d-spacing of the (111) planes of Cu and Co, respectively [36]. Element distribution mappings of selected catalyst for Cu0.5 Co0.5 /PDDA-HNT nanocomposite are presented in Fig. 2f. STEM-energy dispersive X-ray (STEM-EDX) elemental mapping images confirm elements of copper, nitrogen and cobalt uniformly dispersed on the surface of PDDA-HNT. These results reveal that the bimetallic Cu-Co NPs is formed for CuCo/PDDA-HNT nanocomposites. The amounts of metal loading in as-prepared Cux Co1-x /PDDA-HNT (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0) nanocomposites are shown as determined by ICP-MS in Table 1. The mass fraction ratio of elemental copper and cobalt is close to the theoretical metal loading. XPS analysis was performed to investigate the Cu and the Co electronic states on the Cu1.0 /PDDA-HNT, Co1.0 /PDDA-HNT, Cu0.5 Co0.5 /PDDA-HNT surface. Fig. 3a shows the XPS peaks of Cu 2p of the Cu1.0 /PDDA-HNT and Cu0.5 Co0.5 /PDDA-HNT nanocomposites. The observed Cu 2p3/2 and Cu 2p1/2 with binding energies of peaks at 933.1 and 952.6 eV correspond to zerovalent Cu for Cu1.0 /PDDA-HNT, and the slight increase in the binding energies of Cu0.5 Co0.5 /PDDA-HNT reveals that some electrons are transferred from Cu to Co in the Cu0.5 Co0.5 /PDDA-HNT nanocomposite, and it can be understood that there is a possible interaction of some Cu with the Co [3]. Cu 2p3/2 and Cu2p1/2 peaks respectively locate at 933.4 and 953.3 eV belong to Cu+ species [37,38]. Fig. 3b shows the peaks of Co 2p for the Co1.0 /PDDA-HNT and Cu0.5 Co0.5 /PDDAHNT. There are four obvious peaks at 781.8, 787.1, 797.8, and 803.5 eV, which are assigned to 2p3/2 and 2p1/2 of oxidized Co for Co1.0 /PDDA-HNT. The bimetallic Cu-Co NPs can be regarded as the donor-acceptor hybrids. As to Co, the acception of electron from Cu and thereby the change of charge density are revealed by Co 2p XPS spectra. There are even obvious peaks of Co species at 778.6 eV and 793.0 eV for the Cu0.5 Co0.5 /PDDA-HNT, which stand for zerovalent Co [39]. Moreover, the zerovalent Co peaks shift to the lower binding energy compared with that of Co1.0 /PDDA-HNT because some electrons are transferred from Cu to Co in the Cu0.5 Co0.5 /PDDAHNT [3]. This result further confirms the possible interaction of some Cu with the Co [20]. The formation of the Co2+ and Cu+ likely occurs from sampling during the exposure to air [19]. XPS analysis of Cux Co1−x /PDDA-HNT (x = 0, 0.1, 0.3, 0.7, 0.9, 1.0) nanocomposites are shown in Fig. S4. 3.2. Hydrolytic dehydrogenation of AB catalyzed by the Cux Co1−x /PDDA-HNT nanocomposites To investigate the catalytic performances, the synthesized Cux Co1−x /PDDA-HNT nanocomposites with different Cu compositions (x value) have been applied as catalysts for the hydrolytic dehydrogenation of AB. The Cu/Co ratio plays an important role on the particle size of Cu-Co bimetallic nanocatalysts. The sample of Cu0.5 Co0.5 /PDDA-HNT shows the smallest size of 2.2 nm and the excellent dispersion (Fig. 2d) amongst the Cux Co1-x /PDDA-HNT nanocomposites in Fig. S5. The influence of Cu/Co ratio from 0.1 to 0.9 of bimetallic Cu-Co NPs on mean particle size or the TOF value is showed in Fig. 4a. With the increase in Cu molar ratio, the TOF value of the Cux Co1-x /PDDA-HNT system is increasing at first. However,
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Fig. 2. SEM images of HNTs and Cu0.5 Co0.5 /PDDA-HNTs (a, c), HRTEM images of HNTs and Cu0.5 Co0.5 /PDDA-HNTs (b, d), Cu and Co NPs corresponding size histogram (inset (d)); HRTEM lattice fringe image assigned to metal Cu and Co (e), EDX mapping of Cu0.5 Co0.5 /PDDA-HNT (f).
Fig. 3. (a) Cu 2p XPS spectra for Cu1.0 /PDDA-HNT, Cu0.5 Co0.5 /PDDA-HNT, (b) Co 2p XPS spectra for Co1.0 /PDDA-HNT, Cu0.5 Co0.5 /PDDA-HNT samples.
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Fig. 4. Hydrogen generation from the hydrolysis of aqueous AB (0.2 M, 5 mL) at 298 K catalyzed (a) the corresponding TOF value and mean particle size versus Cu molar content (Cu/(Cu + Co)) in CuX Co1-X /PDDA-HNT (x = 0.1, 0.3, 0.5, 0.7, 0.9), (b) Cu0.5 Co0.5 /PDDA-HNT, Cu1.0 /PDDA-HNT, Co1.0 /PDDA-HNT and Cu0.5 Co0.5 /HNT. Table 2 Turnover frequency (TOF) and activation energy (Ea) for different catalysts used for the generation of hydrogen from ammonia borane. Catalysts
T/K
TOF/molH2 min−1 molmetal −1
Ea/kJ mol−1
Ref.
Cu0.5 Co0.5 /C G-Cu36 Ni64 Cu0.3 Co0.7 @MIL-101 Cu0.1 @Co0.45 Ni0.45 /graphene Cu0.2 Co0.8 /C Cu0.9 Co0.1 /rGO Cu0.5 Co0.5 /SiO2 Cu0.2 @Co0.8 -rGO Cu0.5 Co0.5 /PDDA-HNT
298 298 298 298 298 293 298 298 298
28.6 49.1 19.6 15.46 2.09 9.18 9.87 8.36 30.8
51.9 24.4 – 58.41 41.7 – 24 51.3 35.15
3 17 18 40 41 37 19 42 This work
more Co loading would result in the excess coverage of Cu active sites, causing the lower activity for hydrogen generation from the hydrolysis of AB [4]. As shown in Fig. 4a, the Cux Co1-x /PDDA-HNT nanocomposites with a Cu/Co molar ratio of 1: 1 is the most active, giving a total TOF value of 30.8 molH2 /(molmetal · min), which is an impressive value in comparison with other Cu based catalysts recently reported for this reaction in Table 2 [3,17–19,37,40–42]. These results reveal that the chemical composition and size of the metal particles have a significant effect on the catalytic properties of the Cux Co1-x /PDDA-HNT nanocomposites [43]. For further investigating the catalysis activity of the Cu0.5 Co0.5 /PDDA-HNT nanocomposites, the generation of hydrogen from the AB is also conducted under the same conditions using other catalysts for comparison. Fig. 4b shows that Cu1.0 /PDDAHNT, Co1.0 /PDDA-HNT, Cu0.5 Co0.5 /PDDA-HNT and Cu0.5 Co0.5 /HNT nanocomposites are used for hydrolysis of AB (0.2 M, 5 mL) under the same reaction conditions. The Cu1.0 /PDDA-HNT catalysts exhibit a certain activity in the hydrolysis of AB, whereas Co1.0 /PDDA-HNT nanocomposites show a very low catalytic activity for this reaction. Because the Co2+ cannot be easily reduced to Co by using AB as a reducing agent in the present reaction condition (reduction potentials: E0 [Cu2+ ]/[Cu] = + 0.34 eV vs SHE; E0 [Cu+ ]/[Cu] = + 0.52 eV vs SHE; E0 [Co+2 ]/[Co] = − 0.25 eV vs SHE) [19], resulting in a very low catalytic activity for the hydrolytic dehydrogenation of AB [44]. Moreover, AB hydrolysis reaction can be completed (VH2 = 68 mL) in 5.8 min for Cu0.5 Co0.5 /PDDA-HNT nanocomposites at room temperature. In AB catalytic hydrolysis, it is believed that the formation of an intermediate species during the induction time is essential to initiate the reaction. The formation of an activated complex is usually postulated through the interaction of an AB molecule with the surface of the metal catalyst, which then dissociates on attack of a water molecule, readily leading to concerted dissociation of the B–N bonding and hydrolysis of the
resulting BH3 intermediate to produce the borate ion along with the H2 release [45,46]. The existence of Cu and Co nanocatalysts facilitate the formation of the required activated intermediate species. Fig. 4b also shows the catalytic activity comparision of Cu0.5 Co0.5 /HNT and Cu0.5 Co0.5 /PDDA-HNT. The catalytic activity of Cu0.5 Co0.5 /PDDA-HNT is higher than that of Cu0.5 Co0.5 /HNT. Some results revealed that PDDA adsorbed on the surface of supports as glue molecules could facilitate the deposition of more catalysts on supports surface [28–30], and then enhance the rate of hydrolysis dehydrogenation of AB. Compared TEM images of Cu0.5 Co0.5 /PDDA-HNT in Fig. 2d with that of Cu0.5 Co0.5 /HNT in Fig. S6, it could be clearly revealed that the Cu and Co NPs loaded on the original HNTs are less than Cu0.5 Co0.5 /PDDA-HNT.
3.3. Kinetics for hydrolytic dehydrogenation of AB The kinetics of the hydrolysis reaction of AB solution catalyzed by Cu0.5 Co0.5 /PDDA-HNT nanocomposites are further investigated. Fig. 5 shows hydrogen generation rates can be influenced by the temperature, metal amount. In order to study the effect of the temperature on the hydrolysis of AB catalyzed by Cu1.0 /PDDAHNT, Co1.0 /PDDA-HNT, Cu0.5 Co0.5 /PDDA-HNT nanocomposites, a series of experiments have been taken at different reaction temperatures ranging from 298 to 313 K. Fig. 5a shows the plots of mol (H2 /AB) vs reaction time in the hydrolysis of AB catalyzed by Cu0.5 Co0.5 /PDDA-HNT nanocomposites at various temperatures. The hydrogen generation rate increases by increasing the reaction temperature, meanwhile, the TOF value is also increasing from 30.8 to 63.4, which is shown in the inset of Fig. 5a. The values of rate constant k are calculated from the linear portion of each plot in Fig. 5a. The Arrhenius plot of ln k versus 1/T for the nanocomposite is plotted in Fig. 5b. The Ea value of Cu0.5 Co0.5 /PDDA-HNT nanocomposites for the hydrolysis of AB is calculated to be 35.15 kJ mol−1 .
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Fig. 5. (a) Hydrogen generation from the hydrolysis of aqueous AB (0.2 M, 5 mL) catalyzed by Cu0.5 Co0.5 /PDDA-HNT catalyst at 298–313 K (the inset shows the corresponding TOF values). (b) Arrhenius plot: ln k versus 1/T. (c) The effect of metal amount on hydrogen generation rates and (d) The fitting plot of ln k versus ln (metal concentration).
Fig. 6. (a) Recyclability test of Cu0.5 Co0.5 /PDDA-HNT for the hydrolysis of an aqueous AB (0.2 M, 5 mL) solution at 298 K. (b) HRTEM image of Cu0.5 Co0.5 /PDDA-HNT after the 10th catalytic reuse.
And the Ea value of Cu1.0 /PDDA-HNT nanocomposites for the hydrolysis of AB is 37 kJ mol−1 (Fig. S7). Because Co1.0 /PDDAHNT nanocomposites show a very low catalytic activity, Ea value could not be accurately calculated. Table 2 summarizes the Ea and TOF values obtained from the hydrolysis of AB using the prepared catalysts as compared with various Cu based catalysts reported previously. As can be observed, the Ea value for the Cu0.5 Co0.5 /PDDA-HNT nanocomposites in this study was less than those of the several listed catalysts [3,17–19,37,40–42]. The effect of metal concentration on the hydrogen generation for the hydrolysis of AB by Cu0.5 Co0.5 /PDDA-HNT is investigated
by performing a series of control experiments while keeping the other reaction conditions unchanged. Fig. 5c shows the time courses for hydrogen production from hydrolysis of AB by using different metal concentrations. The reaction rates can be determined from the linear portion of each plot. Fig. 5d shows the relation between the reaction rates and the metal concentration in logarithmic scale. It can be seen that the hydrolysis of AB is a first order reaction with respect to the metal concentrations.
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3.4. Recycle test of Cu0.5 Co0.5 /PDDA-HNT for hydrolytic dehydrogenation of AB The catalytic stability of the Cu0.5 Co0.5 /PDDA-HNT nanocomposite in the hydrolytic dehydrogenation of AB is investigated by performing reusability experiments. Fig. 6a shows the hydrogen generation rate and conversion after the completion of the 10th cycle. This used nanocomposites preserved 90% conversion of AB to NH4 BO2 and H2 even at 10th catalytic reuse. The similar decrease phenomenon has also been reported for other bimetallic catalysts [3,15,20], which could be attributed to the precipitation of metaborates to the catalyst surface and the leaching of catalyst into the solution [47,48]. Fig. 6b demonstrates that there are no obvious changes in the morphology for the used Cu0.5 Co0.5 /PDDA-HNTs by HRTEM, which further cofirmed the stability of the nanocomposites for hydrolytic dehydrogenation of AB. 4. Conclusions In summary, non-noble bimetallic Cu-Co/PDDA-HNT nanocomposites have been successfully prepared by using a facile deposition-reduction route at room temperature. Characterization results show that highly dispersed Cu and Co NPs immobilized on the surface PDDA-HNT for the nanocomposite. Cu0.5 Co0.5 /PDDAHNT exhibited excellent catalytic activity with TOF value reaching 30.8 molH2 /(molmetal min−1 ) at the room temperature. The activation energy of the catalyst for the hydrolysis of AB was calculated to be 35.15 kJ mol−1 . The highly efficient nanocomposite represents a promising step toward the practical applications of HNT-supported non-noble metal catalysts in the catalytic hydrolysis reaction system. Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant Nos. 21576247, 21403053 and U1404503). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2017.08. 171. References [1] A. Rossin, M. Peruzzini, Ammonia-borane and amine-borane dehydrogenation mediated by complex metal hydrides, Chem. Rev. 116 (2016) 8848–8872. [2] Y. Ge, Z.H. Shah, X.J. Lin, R. Lu, Z. Liao, S. Zhang, Highly efficient Pt decorated CoCu bimetallic nanoparticles protected in silica for hydrogen production from ammonia-borane, ACS Sustainable Chem. Eng. 5 (2016) 1675–1684. [3] A. Bulut, M. Yurderi, I˙ .E. Ertas, M. Celebi, M. Kaya, M. Zahmakiran, Carbon dispersed copper-cobalt alloy nanoparticles: a cost-effective heterogeneous catalyst with exceptional performance in the hydrolytic dehydrogenation of ammonia-borane, Appl. Catal. B 180 (2016) 121–129. [4] 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. [5] Y. Hu, Y. Wang, Z.H. Lu, X. Chen, L. Xiong, Core-shell nanospheres Pt@SiO2 for catalytic hydrogen production, Appl. Surf. Sci. 341 (2015) 185–189. [6] Y. Yang, Z.H. Lu, Y. Hu, Z. Zhang, W. Shi, X. Chen, T. Wang, Facile in situ synthesis of copper nanoparticles supported on reduced graphene oxide for hydrolytic dehydrogenation of ammonia borane, RSC Adv. 4 (2014) 13749–13752. [7] M.A. Khalily, H. Eren, S. Akbayrak, H.H. Susapto, N. Biyikli, S. Ozkar, M.O. Guler, Facile synthesis of three-dimensional Pt-TiO2 nano-networks: a highly active catalyst for the hydrolytic dehydrogenation of ammonia-borane, Angew. Chem. 128 (2016) 12445–12449. [8] A. Aijaz, A. Karkamkar, Y.J. Choi, N. Tsumori, E. Ronnebro, T. Autrey, H. Shioyama, Q. Xu, Immobilizing highly catalytically active Pt nanoparticles inside the pores of metal-organic framework: a double solvents approach, J. Am. Chem. Soc. 134 (2012) 13926–13929.
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Full Length Article
Preparation of bimetallic Cu-Co nanocatalysts on poly (diallyldimethylammonium chloride) functionalized halloysite nanotubes for hydrolytic dehydrogenation of ammonia borane Yang Liu, Jun Zhang, Huijuan Guan, Yafei Zhao, Jing-He Yang ∗ , Bing Zhang ∗ School of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou, 450001, PR China
a r t i c l e
i n f o
Article history: Received 7 June 2017 Received in revised form 23 August 2017 Accepted 24 August 2017 Keywords: Halloysite nanotubes Ammonia borane Bimetallic copper-cobalt Hydrolytic dehydrogenation Polyelectrolytes modification
a b s t r a c t In present work, we prepared the bimetallic Cu-Co nanocatalysts on poly (diallyldimethylammonium chloride) functionalized halloysite nanotubes (Cu-Co/PDDA-HNTs) by a deposition-reduction technique at room temperature. The analysis of XRD, SEM, TEM, HAADF-STEM and XPS were employed to systematically investigate the morphology, particle size, structure and surface properties of the nanocomposite. The results reveal that the PDDA coating with thickness of ∼15 nm could be formed on the surface of HNTs, and the existence of PDDA is beneficial to deposit Cu and Co nanoparticles (NPs) with high dispersibility on the surface. While the cost-effective nanocomposite was used for the hydrolytic dehydrogenation of ammonia-borane (NH3 BH3 ), the nanocatalyst showed extraordinary catalytic properties with high total turnover frequency of 30.8 molH2 /(molmetal min), low activation energy of 35.15 kJ mol−1 and high recycling stability (>90% conversion at 10th reuse). These results indicate that the bimetallic Cu-Co nanocatalysts on PDDA functionalized HNTs have particular potential for application in release hydrogen process. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen is considered to be one of the most promising energy carriers for the future owing to its clean burning nature, environmental friendliness, and high energy content. However, the hydrogen storage method that can release hydrogen safely and efficiently under ambient condition still faces many challenges for the future “hydrogen economy” [1,2]. Among the new hydrogen storage materials, NH3 BH3 (AB) appears to be the most promising material because of its low molecular weight (30.9 g mol−1 ), nontoxicity, and high hydrogen density (19.6 wt%), which is greater than the target value of U.S. Department of Energy (5.5 wt% H2 ) [3–5]. More importantly, with an appropriate catalyst, catalytic hydrolysis of AB can release 3 mol H2 per mol AB at room temperature [6]. So far, a wide range of catalysts based mostly on transition-metal NPs have been developed for the hydrolytic dehydrogenation of AB including noble metals and non-noble metals such as Pt, Ru, Pd, Co, Cu and Ni [6–14]. Bimetallic nanocatalysts usually have higher cat-
∗ Corresponding author. E-mail addresses: [email protected] (J.-H. Yang), [email protected] (B. Zhang). http://dx.doi.org/10.1016/j.apsusc.2017.08.171 0169-4332/© 2017 Elsevier B.V. All rights reserved.
alytic performance than their monometallic counterparts, owing to the synergistic geometric and electronic effects of the bimetallic NPs, such as Co-Pd, Ni-Mo and Cu-Ni, Cu-Co [15–20]. To be active in the catalytic reaction, metal catalysts are required to have an ultrafine and uniform size, which cause the increase in the number of active sites. However, these metallic NPs often suffer from insufficient stability/durability owing to agglomeration [21]. Recently, solid support materials have received considerable attention, which could effectively restrain the agglomeration of metal NPs and enhance the catalytic activities. Meanwhile, the supports could limit the migration of the active components and thus enhance the long-term stability [22]. Halloysite nanotubes [Al2 Si2 O5 (OH)4 ·nH2 O, HNTs] are cheap and have an abundant natural source in many countries, such as Australia, New Zealand, China, Guyana, Mexico, and Brazil. HNTs consist of a 1:1 layered aluminosilicate mineral that has a predominantly hollow tubular structure, which offer potential applications as supports for catalytic composites with distinctly improved stability and mechanical property [23–25]. However, natural HNTs exhibit a low adhesion for bonding metal NPs because of the absence of chemical conjunction, which leads to easy leaching of the metal particles from the surface during reactions [26]. To solve this problem, one of the methods is the effective surface functionalization of HNTs, which makes it possible to improve adhesive
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capacity of metal NPs on their surface. PDDA, a water soluble quaternary ammonium polyelectrolyte, is renowned as an environmentally friendly and low cost polyelectrolyte [27]. Previous studies reported the use of PDDA as a reducing agent and a stabilizer for fabricating nanostructured metals. In addition, PDDA also can be utilized to prepare composite materials via surface modification of carbon-based materials (e.g. graphene and carbon nanotubes) for subsequent NP deposition [28–30]. However, up to now, functionalized HNT with PDDA as the substrate for the catalytic field has been rarely reported. In this work, we immobilized the bimetallic Cu-Co NPs on HNT decorated with PDDA by a facile deposition-reduction method, and employed it as catalyst for hydrogen generation from the hydrolysis of AB at room temperature. We found that the synthesized Cu-Co/PDDA-HNT nanocomposites exhibit higher catalytic activity, in comparison to their monometallic counterparts. Especially, Cu0.5 Co0.5 /PDDA-HNT shows the best catalytic activities among the Cux Co1−x /PDDA-HNT system, a high turnover frequency, a low activation energy and a good recycling stability in the hydrolytic dehydrogenation of AB at room temperature. 2. Experimental 2.1. Chemicals All chemical reagents were obtained from commercial suppliers and used without further purification. Halloysite nanotubes (HNTs) were refined from clay minerals in Henan province, China. Poly (diallyldimethylammonium chloride) (PDDA, Mw = 100,000–200,000) in 20% aqueous solution, cobalt (II) sulfate heptahydrate (CoSO4 ·7H2 O, 99%) and ammonia borane complex (NH3 BH3 , AB, 97%) were purchased from Aldrich, copper (II) chloride dehydrate (CuCl2 ·2H2 O, 99%) was purchased from Tianjin Fengchuan Chemical Reagent Co. Ltd. China., sodium borohydride (NaBH4 , 97%) was purchased from Sinopharm Chemical Reagent Co. Ltd. China. All other regents were of analytical reagent grade. The water was used deionized water.
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followed the same process. The metal content of the product (Cu + Co)/(Cu + Co + PDDA-HNT) was kept at about 10 wt%. 2.4. Determination of the catalytic activity of Cu-Co/PDDA-HNT nanocomposites in the hydrolytic dehydrogenation of AB The hydrolysis equation of AB is briefly presented as follows: NH3 BH3 + 2H2 O → NH4 + + BO2 − + 3H2 ↑ The volume of released gas was monitored using the gas burette by water displacement during the AB catalyzed by the catalysts. In a typical experiment, a three necked reaction flask (25.0 mL) containing the as-synthesized nanocomposites suspension (4.0 mL) was placed on a magnetic stirrer and thermostated to 298 K by using a constant temperature bath. The left and middle necks were sealed with the glass stoppers, the right neck was sealed with the glass stopper which was connected to the gas-collection tube. After the suspension was stirred 15 min, 1.0 mL aqueous solution of 32 mg AB was added into the reaction flask by the middle neck then kept in the glass stopper. The flask was well sealed with vaseline. The reaction time was recorded from the appearance of the first bubble. The reaction was finished when there was no more gas evolved. Finally, the catalytic activity of Cu-Co/PDDA-HNT samples in the hydrolysis of AB was evaluated by the turnover frequency (TOF) value. According to the references [32,33], the TOF numbers are calculated by a typical formula: TOF = nH2 /(nmetal × t), in which nH2 is the mole number of generated H2 , while nmetal is the total mole number of metal (Cu and Co), t is the completed reaction time (min). The catalytic reactions were also carried out at different temperatures ranging from 298 to 313 K under an ambient atmosphere to get the activation energy (Ea) of the AB hydrolysis catalyzed by CuCo/PDDA-HNT nanocomposites. The Ea is calculated according to the following Arrhenius equation [11]: lnk = lnA − Ea/RT
2.2. Synthesis of PDDA-HNTs PDDA/HNTs were prepared according to the reported method with a slight modification [31]. Briefly, 0.5 g of HNTs was dispersed into 50.0 mL of a 2.5 wt% PDDA aqueous solution containing 0.5 M NaCl and ultrasonicated for 1 h to obtain a homogeneous white suspension and then rapid magnetic stirring for 24 h was used to ensure the presence of well dispersed PDDA-HNTs. The dispersion was centrifuged and washed with water several times to remove redundant PDDA and then the collected precipitate was dried under vacuum at room temperature to get the PDDA-HNTs.
2.5. Reusability performance of Cu0.5 Co0.5 /PDDA-HNT nanocomposite in the hydrolytic dehydrogenation of AB After the first run of hydrolytic dehydrogenation of AB solution (0.2 M, 5 mL) starting with Cu0.5 Co0.5 /PDDA-HNT at 298 K, the catalyst was isolated from reaction solution by centrifugation and washed with excess water, then dried in vacuum oven at room temperature. The dried nanocomposite was used again in the hydrolytic dehydrogenation of AB solution (0.2 M, 5 mL) at 298 K. Such a recycle test of the composites for the hydrolysis of AB was carried out for 10 runs under an ambient atmosphere at room temperature.
2.3. Preparation of Cu-Co/PDDA-HNT nanocomposites 2.6. Characterization Cu0.5 Co0.5 /PDDA-HNT nanocomposites were synthesized as follows: 50.0 mg of the prepared PDDA/HNTs was dispersed in 15.0 mL aqueous solution, to which 5.0 mL of aqueous solution containing 7.8 mg CuCl2 ·2H2 O and 12.8 mg CoSO4 ·7H2 O. After stirring at room temperature for 2 h, 5.0 mL aqueous solution of 55.6 mg NaBH4 was added by drop to the above mixture and the resulting solution was stirred for half an hour under air at room temperature. After centrifugation (8000 rpm, 5 min), copious washing with water, filtration, and drying in vacuum oven at room temperature. Cu0.5 Co0.5 /PDDA-HNT nanocomposite was obtained as green powders and stored in vacuum oven at room temperature. The synthesis of other Cux Co1-x /PDDA-HNT nanocomposites with different Cu molar content (x = 0, 0.1, 0.3, 0.7, 0.9, 1.0)
The surface morphology of the nanocatalysts were characterized by scanning electron microscopy (SEM, Hitachi S2400). The morphology and crystal structure of the samples were observed by high resolution transmission electron microscopy (HRTEM) and high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) on a FEI Talosf200S operating at an acceleration voltage of 200 kV. The crystal structures of the products were characterized by X-ray diffraction (XRD) on a D8ADVANCE in the range of 5–75◦ . The chemical compositions of the catalysts were analyzed by using ELAN 9000 inductively coupled plasma atomic mass spectrometer (ICP-MS). X-ray photoelectron spectroscopy (XPS) measurement was performed with
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Y. Liu et al. / Applied Surface Science 427 (2018) 106–113 Table 1 ICP-MS analysis of Cux Co1-x /PDDA-HNT nanocomposites.
Fig. 1. The X-ray diffraction (XRD) patterns of (a) HNT, (b) PDDA-HNT, (c) Cu0.5 Co0.5 /PDDA-HNT.
a Thermo Scientific-ESCALAB 250XI multifunctional imaging electron spectrometer to study the surface properties of the catalysts. 3. Results and discussion 3.1. Characterization of Cu-Co/PDDA-HNT nanocomposite Fig. 1 depicts XRD patterns of HNT, PDDA-HNT, and Cu0.5 Co0.5 /PDDA-HNT materials. In the XRD pattern of HNTs, all of the observed peaks can be indexed to the characteristic peaks of halloysite (JCPDS No.09-0453) [34]. The peak appearing at 2 of 11.79◦ , 20.07◦ , 24.50◦ , 35.02◦ correspond to (001), (100), (002), (110) crystal planes respectively are clearly observed in Fig. 1a. The PDDA-HNT and HNT show similar peaks in Fig. 1b [35]. Apart from the reflection peaks of halloysite itself, two additional intense peaks are observed for Cu0.5 Co0.5 /PDDA-HNT nanoconposite at 43.3◦ , 36.5◦ , which can be assigned to Cu [111] (JCPDS No.04-0836) and Cu2 O [111] (JCPDS No.05-0667). The main reason for the observation of cuprous oxide phases is the high oxophilicity of copper (0), which is due to the partly surface oxidation of Cu NPs and could not be avoided [3]. However, the Co NPs are not observed from XRD in the sample, which may be due to the fact that the cobalt NPs are too small or amorphous [19,36]. XRD patterns of Cux Co1-x /PDDA-HNT (x = 0, 0.1, 0.3, 0.7, 0.9, 1.0) nanocomposites are shown in Fig. S1. SEM and TEM images of HNTs and Cu0.5 Co0.5 /PDDA-HNTs are presented in Fig. 2a–d. The structure and morphology of the natural HNTs in Fig. 2a and b. It can be seen that natural HNTs mainly consist of hollow nanotubes in various sizes. The HNTs have lengths ranging from 0.5 to 1 m, diameters in the range of 40–60 nm. Fig. 2c and d exhibits morphology and structure of Cu0.5 Co0.5 /PDDA-HNTs. The distinct morphology shows that PDDA layer has successfully coated on surface of HNTs in Fig. S2 and the Cu0.5 Co0.5 /PDDA-HNTs nanocomposites have a slight agglomeration in Fig. 2c. Fig. 2d shows the tubular structure disappears, PDDA layer may diffuse into the inner surface of HNTs. We also notice that highly dispersed ultrafine NPs immobilized on the outer layer. The HAADF-STEM images of Cu0.5 Co0.5 /PDDA-HNTs in Fig. S3 also show the ultrafine NPs immobilized on the surface of the PDDAHNTs and the PDDA coating with thickness of ∼15 nm could be found from the surface of HNTs. Moreover, the size distribution of NPs for Cu0.5 Co0.5 /PDDA-HNT is estimated by Nano Measurer software, as shown in inset of Fig. 2d. The mean metal NPs size is found to be ∼2.2 nm. Additionally, the structural characterization of Cu and Co NPs decorating the PDDA-HNT in further detail is achieved using HRTEM in Fig. 2e. The lattice fringes with spacing of 0.209 nm and 0.200 nm are clearly visible in NPs on PDDA-
Catalysts
Cu loading wt%
Co1.0 /PDDA-HNT Cu0.1 Co0.9 /PDDA-HNT Cu0.3 Co0.7 /PDDA-HNT Cu0.5 Co0.5 /PDDA-HNT Cu0.7 Co0.3 /PDDA-HNT Cu0.9 Co0.1 /PDDA-HNT Cu1.0 /PDDA-HNT
1.0 2.9 4.4 7.0 8.7 9.7
Co loading wt% 9.7 8.3 6.3 4.6 2.7 0.7
HNT, which are consistent with the d-spacing of the (111) planes of Cu and Co, respectively [36]. Element distribution mappings of selected catalyst for Cu0.5 Co0.5 /PDDA-HNT nanocomposite are presented in Fig. 2f. STEM-energy dispersive X-ray (STEM-EDX) elemental mapping images confirm elements of copper, nitrogen and cobalt uniformly dispersed on the surface of PDDA-HNT. These results reveal that the bimetallic Cu-Co NPs is formed for CuCo/PDDA-HNT nanocomposites. The amounts of metal loading in as-prepared Cux Co1-x /PDDA-HNT (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0) nanocomposites are shown as determined by ICP-MS in Table 1. The mass fraction ratio of elemental copper and cobalt is close to the theoretical metal loading. XPS analysis was performed to investigate the Cu and the Co electronic states on the Cu1.0 /PDDA-HNT, Co1.0 /PDDA-HNT, Cu0.5 Co0.5 /PDDA-HNT surface. Fig. 3a shows the XPS peaks of Cu 2p of the Cu1.0 /PDDA-HNT and Cu0.5 Co0.5 /PDDA-HNT nanocomposites. The observed Cu 2p3/2 and Cu 2p1/2 with binding energies of peaks at 933.1 and 952.6 eV correspond to zerovalent Cu for Cu1.0 /PDDA-HNT, and the slight increase in the binding energies of Cu0.5 Co0.5 /PDDA-HNT reveals that some electrons are transferred from Cu to Co in the Cu0.5 Co0.5 /PDDA-HNT nanocomposite, and it can be understood that there is a possible interaction of some Cu with the Co [3]. Cu 2p3/2 and Cu2p1/2 peaks respectively locate at 933.4 and 953.3 eV belong to Cu+ species [37,38]. Fig. 3b shows the peaks of Co 2p for the Co1.0 /PDDA-HNT and Cu0.5 Co0.5 /PDDAHNT. There are four obvious peaks at 781.8, 787.1, 797.8, and 803.5 eV, which are assigned to 2p3/2 and 2p1/2 of oxidized Co for Co1.0 /PDDA-HNT. The bimetallic Cu-Co NPs can be regarded as the donor-acceptor hybrids. As to Co, the acception of electron from Cu and thereby the change of charge density are revealed by Co 2p XPS spectra. There are even obvious peaks of Co species at 778.6 eV and 793.0 eV for the Cu0.5 Co0.5 /PDDA-HNT, which stand for zerovalent Co [39]. Moreover, the zerovalent Co peaks shift to the lower binding energy compared with that of Co1.0 /PDDA-HNT because some electrons are transferred from Cu to Co in the Cu0.5 Co0.5 /PDDAHNT [3]. This result further confirms the possible interaction of some Cu with the Co [20]. The formation of the Co2+ and Cu+ likely occurs from sampling during the exposure to air [19]. XPS analysis of Cux Co1−x /PDDA-HNT (x = 0, 0.1, 0.3, 0.7, 0.9, 1.0) nanocomposites are shown in Fig. S4. 3.2. Hydrolytic dehydrogenation of AB catalyzed by the Cux Co1−x /PDDA-HNT nanocomposites To investigate the catalytic performances, the synthesized Cux Co1−x /PDDA-HNT nanocomposites with different Cu compositions (x value) have been applied as catalysts for the hydrolytic dehydrogenation of AB. The Cu/Co ratio plays an important role on the particle size of Cu-Co bimetallic nanocatalysts. The sample of Cu0.5 Co0.5 /PDDA-HNT shows the smallest size of 2.2 nm and the excellent dispersion (Fig. 2d) amongst the Cux Co1-x /PDDA-HNT nanocomposites in Fig. S5. The influence of Cu/Co ratio from 0.1 to 0.9 of bimetallic Cu-Co NPs on mean particle size or the TOF value is showed in Fig. 4a. With the increase in Cu molar ratio, the TOF value of the Cux Co1-x /PDDA-HNT system is increasing at first. However,
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Fig. 2. SEM images of HNTs and Cu0.5 Co0.5 /PDDA-HNTs (a, c), HRTEM images of HNTs and Cu0.5 Co0.5 /PDDA-HNTs (b, d), Cu and Co NPs corresponding size histogram (inset (d)); HRTEM lattice fringe image assigned to metal Cu and Co (e), EDX mapping of Cu0.5 Co0.5 /PDDA-HNT (f).
Fig. 3. (a) Cu 2p XPS spectra for Cu1.0 /PDDA-HNT, Cu0.5 Co0.5 /PDDA-HNT, (b) Co 2p XPS spectra for Co1.0 /PDDA-HNT, Cu0.5 Co0.5 /PDDA-HNT samples.
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Fig. 4. Hydrogen generation from the hydrolysis of aqueous AB (0.2 M, 5 mL) at 298 K catalyzed (a) the corresponding TOF value and mean particle size versus Cu molar content (Cu/(Cu + Co)) in CuX Co1-X /PDDA-HNT (x = 0.1, 0.3, 0.5, 0.7, 0.9), (b) Cu0.5 Co0.5 /PDDA-HNT, Cu1.0 /PDDA-HNT, Co1.0 /PDDA-HNT and Cu0.5 Co0.5 /HNT. Table 2 Turnover frequency (TOF) and activation energy (Ea) for different catalysts used for the generation of hydrogen from ammonia borane. Catalysts
T/K
TOF/molH2 min−1 molmetal −1
Ea/kJ mol−1
Ref.
Cu0.5 Co0.5 /C G-Cu36 Ni64 Cu0.3 Co0.7 @MIL-101 Cu0.1 @Co0.45 Ni0.45 /graphene Cu0.2 Co0.8 /C Cu0.9 Co0.1 /rGO Cu0.5 Co0.5 /SiO2 Cu0.2 @Co0.8 -rGO Cu0.5 Co0.5 /PDDA-HNT
298 298 298 298 298 293 298 298 298
28.6 49.1 19.6 15.46 2.09 9.18 9.87 8.36 30.8
51.9 24.4 – 58.41 41.7 – 24 51.3 35.15
3 17 18 40 41 37 19 42 This work
more Co loading would result in the excess coverage of Cu active sites, causing the lower activity for hydrogen generation from the hydrolysis of AB [4]. As shown in Fig. 4a, the Cux Co1-x /PDDA-HNT nanocomposites with a Cu/Co molar ratio of 1: 1 is the most active, giving a total TOF value of 30.8 molH2 /(molmetal · min), which is an impressive value in comparison with other Cu based catalysts recently reported for this reaction in Table 2 [3,17–19,37,40–42]. These results reveal that the chemical composition and size of the metal particles have a significant effect on the catalytic properties of the Cux Co1-x /PDDA-HNT nanocomposites [43]. For further investigating the catalysis activity of the Cu0.5 Co0.5 /PDDA-HNT nanocomposites, the generation of hydrogen from the AB is also conducted under the same conditions using other catalysts for comparison. Fig. 4b shows that Cu1.0 /PDDAHNT, Co1.0 /PDDA-HNT, Cu0.5 Co0.5 /PDDA-HNT and Cu0.5 Co0.5 /HNT nanocomposites are used for hydrolysis of AB (0.2 M, 5 mL) under the same reaction conditions. The Cu1.0 /PDDA-HNT catalysts exhibit a certain activity in the hydrolysis of AB, whereas Co1.0 /PDDA-HNT nanocomposites show a very low catalytic activity for this reaction. Because the Co2+ cannot be easily reduced to Co by using AB as a reducing agent in the present reaction condition (reduction potentials: E0 [Cu2+ ]/[Cu] = + 0.34 eV vs SHE; E0 [Cu+ ]/[Cu] = + 0.52 eV vs SHE; E0 [Co+2 ]/[Co] = − 0.25 eV vs SHE) [19], resulting in a very low catalytic activity for the hydrolytic dehydrogenation of AB [44]. Moreover, AB hydrolysis reaction can be completed (VH2 = 68 mL) in 5.8 min for Cu0.5 Co0.5 /PDDA-HNT nanocomposites at room temperature. In AB catalytic hydrolysis, it is believed that the formation of an intermediate species during the induction time is essential to initiate the reaction. The formation of an activated complex is usually postulated through the interaction of an AB molecule with the surface of the metal catalyst, which then dissociates on attack of a water molecule, readily leading to concerted dissociation of the B–N bonding and hydrolysis of the
resulting BH3 intermediate to produce the borate ion along with the H2 release [45,46]. The existence of Cu and Co nanocatalysts facilitate the formation of the required activated intermediate species. Fig. 4b also shows the catalytic activity comparision of Cu0.5 Co0.5 /HNT and Cu0.5 Co0.5 /PDDA-HNT. The catalytic activity of Cu0.5 Co0.5 /PDDA-HNT is higher than that of Cu0.5 Co0.5 /HNT. Some results revealed that PDDA adsorbed on the surface of supports as glue molecules could facilitate the deposition of more catalysts on supports surface [28–30], and then enhance the rate of hydrolysis dehydrogenation of AB. Compared TEM images of Cu0.5 Co0.5 /PDDA-HNT in Fig. 2d with that of Cu0.5 Co0.5 /HNT in Fig. S6, it could be clearly revealed that the Cu and Co NPs loaded on the original HNTs are less than Cu0.5 Co0.5 /PDDA-HNT.
3.3. Kinetics for hydrolytic dehydrogenation of AB The kinetics of the hydrolysis reaction of AB solution catalyzed by Cu0.5 Co0.5 /PDDA-HNT nanocomposites are further investigated. Fig. 5 shows hydrogen generation rates can be influenced by the temperature, metal amount. In order to study the effect of the temperature on the hydrolysis of AB catalyzed by Cu1.0 /PDDAHNT, Co1.0 /PDDA-HNT, Cu0.5 Co0.5 /PDDA-HNT nanocomposites, a series of experiments have been taken at different reaction temperatures ranging from 298 to 313 K. Fig. 5a shows the plots of mol (H2 /AB) vs reaction time in the hydrolysis of AB catalyzed by Cu0.5 Co0.5 /PDDA-HNT nanocomposites at various temperatures. The hydrogen generation rate increases by increasing the reaction temperature, meanwhile, the TOF value is also increasing from 30.8 to 63.4, which is shown in the inset of Fig. 5a. The values of rate constant k are calculated from the linear portion of each plot in Fig. 5a. The Arrhenius plot of ln k versus 1/T for the nanocomposite is plotted in Fig. 5b. The Ea value of Cu0.5 Co0.5 /PDDA-HNT nanocomposites for the hydrolysis of AB is calculated to be 35.15 kJ mol−1 .
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Fig. 5. (a) Hydrogen generation from the hydrolysis of aqueous AB (0.2 M, 5 mL) catalyzed by Cu0.5 Co0.5 /PDDA-HNT catalyst at 298–313 K (the inset shows the corresponding TOF values). (b) Arrhenius plot: ln k versus 1/T. (c) The effect of metal amount on hydrogen generation rates and (d) The fitting plot of ln k versus ln (metal concentration).
Fig. 6. (a) Recyclability test of Cu0.5 Co0.5 /PDDA-HNT for the hydrolysis of an aqueous AB (0.2 M, 5 mL) solution at 298 K. (b) HRTEM image of Cu0.5 Co0.5 /PDDA-HNT after the 10th catalytic reuse.
And the Ea value of Cu1.0 /PDDA-HNT nanocomposites for the hydrolysis of AB is 37 kJ mol−1 (Fig. S7). Because Co1.0 /PDDAHNT nanocomposites show a very low catalytic activity, Ea value could not be accurately calculated. Table 2 summarizes the Ea and TOF values obtained from the hydrolysis of AB using the prepared catalysts as compared with various Cu based catalysts reported previously. As can be observed, the Ea value for the Cu0.5 Co0.5 /PDDA-HNT nanocomposites in this study was less than those of the several listed catalysts [3,17–19,37,40–42]. The effect of metal concentration on the hydrogen generation for the hydrolysis of AB by Cu0.5 Co0.5 /PDDA-HNT is investigated
by performing a series of control experiments while keeping the other reaction conditions unchanged. Fig. 5c shows the time courses for hydrogen production from hydrolysis of AB by using different metal concentrations. The reaction rates can be determined from the linear portion of each plot. Fig. 5d shows the relation between the reaction rates and the metal concentration in logarithmic scale. It can be seen that the hydrolysis of AB is a first order reaction with respect to the metal concentrations.
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3.4. Recycle test of Cu0.5 Co0.5 /PDDA-HNT for hydrolytic dehydrogenation of AB The catalytic stability of the Cu0.5 Co0.5 /PDDA-HNT nanocomposite in the hydrolytic dehydrogenation of AB is investigated by performing reusability experiments. Fig. 6a shows the hydrogen generation rate and conversion after the completion of the 10th cycle. This used nanocomposites preserved 90% conversion of AB to NH4 BO2 and H2 even at 10th catalytic reuse. The similar decrease phenomenon has also been reported for other bimetallic catalysts [3,15,20], which could be attributed to the precipitation of metaborates to the catalyst surface and the leaching of catalyst into the solution [47,48]. Fig. 6b demonstrates that there are no obvious changes in the morphology for the used Cu0.5 Co0.5 /PDDA-HNTs by HRTEM, which further cofirmed the stability of the nanocomposites for hydrolytic dehydrogenation of AB. 4. Conclusions In summary, non-noble bimetallic Cu-Co/PDDA-HNT nanocomposites have been successfully prepared by using a facile deposition-reduction route at room temperature. Characterization results show that highly dispersed Cu and Co NPs immobilized on the surface PDDA-HNT for the nanocomposite. Cu0.5 Co0.5 /PDDAHNT exhibited excellent catalytic activity with TOF value reaching 30.8 molH2 /(molmetal min−1 ) at the room temperature. The activation energy of the catalyst for the hydrolysis of AB was calculated to be 35.15 kJ mol−1 . The highly efficient nanocomposite represents a promising step toward the practical applications of HNT-supported non-noble metal catalysts in the catalytic hydrolysis reaction system. Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant Nos. 21576247, 21403053 and U1404503). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2017.08. 171. References [1] A. Rossin, M. Peruzzini, Ammonia-borane and amine-borane dehydrogenation mediated by complex metal hydrides, Chem. Rev. 116 (2016) 8848–8872. [2] Y. Ge, Z.H. Shah, X.J. Lin, R. Lu, Z. Liao, S. Zhang, Highly efficient Pt decorated CoCu bimetallic nanoparticles protected in silica for hydrogen production from ammonia-borane, ACS Sustainable Chem. Eng. 5 (2016) 1675–1684. [3] A. Bulut, M. Yurderi, I˙ .E. Ertas, M. Celebi, M. Kaya, M. Zahmakiran, Carbon dispersed copper-cobalt alloy nanoparticles: a cost-effective heterogeneous catalyst with exceptional performance in the hydrolytic dehydrogenation of ammonia-borane, Appl. Catal. B 180 (2016) 121–129. [4] 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. [5] Y. Hu, Y. Wang, Z.H. Lu, X. Chen, L. Xiong, Core-shell nanospheres Pt@SiO2 for catalytic hydrogen production, Appl. Surf. Sci. 341 (2015) 185–189. [6] Y. Yang, Z.H. Lu, Y. Hu, Z. Zhang, W. Shi, X. Chen, T. Wang, Facile in situ synthesis of copper nanoparticles supported on reduced graphene oxide for hydrolytic dehydrogenation of ammonia borane, RSC Adv. 4 (2014) 13749–13752. [7] M.A. Khalily, H. Eren, S. Akbayrak, H.H. Susapto, N. Biyikli, S. Ozkar, M.O. Guler, Facile synthesis of three-dimensional Pt-TiO2 nano-networks: a highly active catalyst for the hydrolytic dehydrogenation of ammonia-borane, Angew. Chem. 128 (2016) 12445–12449. [8] A. Aijaz, A. Karkamkar, Y.J. Choi, N. Tsumori, E. Ronnebro, T. Autrey, H. Shioyama, Q. Xu, Immobilizing highly catalytically active Pt nanoparticles inside the pores of metal-organic framework: a double solvents approach, J. Am. Chem. Soc. 134 (2012) 13926–13929.
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