Amorphous NiP supported on rGO for superior

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Amorphous NiP supported on rGO for superior hydrogen generation from hydrolysis of ammonia borane Xiaoqiong Du a, Chenlu Yang a, Xiang Zeng a, Tong Wu a, Yinghui Zhou c, Ping Cai a, Gongzhen Cheng a, Wei Luo a,b,* a

College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei, 430072, PR China Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin, 300071, PR China c Kingfa SCI. & Tech. CO. LTD., Guangzhou, 510663, PR China b

article info

abstract

Article history:

Transition metal phosphide based amorphous NiP/rGO hybrids have been successfully

Received 5 January 2017

synthesized through a facile one-pot co-reduction method. The prepared NiP/rGO hybrids

Received in revised form

are characterized by powder X-ray diffraction (XRD), inductively coupled plasma-atomic

7 April 2017

emission spectroscopy (ICP-AES), X-ray photoelectron spectroscopy (XPS), transmission

Accepted 9 April 2017

electron microscope (TEM) and energy dispersive X-ray detector (EDX) techniques.

Available online 2 May 2017

Compared with Ni/rGO, the resulted Ni91P9/rGO hybrid exhibits superior catalytic activity toward hydrogen generation from hydrolysis of ammonia borane (NH3BH3), with turnover

Keywords:

frequency (TOF) value of 13.3 min1. The activation energy (Ea) of Ni91P9/rGO for this re-

Transition metal phosphides

action is calculated to be 34.7 kJ mol1, which is much lower than those of the other re-

NiP/rGO

ported catalysts. This superior catalytic performance may be due to the good dispersibility

Hydrogen storage

of Ni91P9 nanoparticles and the synergistic electronic interactions between nickel and

Ammonia borane

phosphorus. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Hydrogen, which makes water vapor upon combustion, has been proposed as an ideal energy carrier to replace traditional fossil fuels due to its high energy density, environmental benignity, and ability to be made from renewable energy sources [1e5]. However, the safe and efficient storage/transport hydrogen is still the main challenge for the widespread “hydrogen economy”. Till now, a number of hydrogen storage materials, including metal hydrides [6,7], absorption materials

[8,9], and chemical hydrides have been widely investigated [10e13]. Ammonia borane (NH3BH3, AB), with high gravimetric hydrogen content (19.6 wt.%) and high stability in aqueous solution, has been considered as one of the most attractive and promising candidates for the portable hydrogen storage application among other practical hydrogen storage materials [14e16]. As shown in Eq. (1), the hydrolysis of AB can release 3 mol H2 per mol AB in the presence of appropriate catalysts [17]. Precious metals, such as Pt [18,19], Rh [20], and Ru [21,22], are the most active catalysts for this reaction, however, their high cost and scarcity severely hinder their widespread

* Corresponding author. College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei, 430072, PR China. E-mail address: [email protected] (W. Luo). http://dx.doi.org/10.1016/j.ijhydene.2017.04.052 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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applications. Thus, the development of cost-effective, active, and durable catalysts for AB hydrolysis is highly desirable, but still a great challenge [23e25]. catalyst

NH3 BH3 þ2H2 O !NH4 BO2 þ3H2

atmosphere. After 4 h, the resultant solution was filtered by suction filtration and the filtrate was concentrated under vacuum at room temperature to obtain the crude product. The final product was further purified by diethyl ether.

(1)

Preparation of graphene oxide (GO) On the other hand, transition metal phosphides (TMPs) with unique charged natures (positive charge in transition metal and negative charge in phosphorus) [26e28], have attracted special attention in hydrodesulphurization (HDS) and overall water splitting [29e34], probably due to the high catalytic activity derived from their hydrogenase-like catalytic mechanisms. However, the development of highly efficient TMPs toward catalytic hydrolysis of AB has not been widely studied. Very recently, Fu and co-workers first reported the synthesis of nanostructured Ni2P by reacting Ni(OH)2 powders with NaH2PO2 in argon at 543 K, and its superior catalytic activity toward hydrolysis of AB [35]. Mechanism study indicated the combination of catalyst's surface and substrate molecules is the key effect for the enhanced catalytic activity. In this study, we report a facile one-pot synthesis of reduced graphene oxide (rGO) supported amorphous NiP nanoparticles (NPs) (NiP/rGO), and their superior catalytic activities toward hydrolytic dehydrogenation of AB. Unexpectedly, the resulted NiP/rGO catalyst exhibits an initial turnover frequency (TOF) 1 for hydrogen generation value of 13.3 mol(H2) mol1 (Ni) min from AB, which is about 3 times higher than that of Ni/rGO 1 (TOF value is about 3.8 mol(H2) mol1 (Ni) min ) without P doping.

Graphene oxide (GO) was prepared according to the modified Hummers method [36]. In an improved synthesis of graphene oxide, 9:1 mixture of concentrated H2SO4/H3PO4 (360: 40 mL) was added to a mixture of graphite ﬂakes (3.0 g) and KMnO4 (18.0 g). The reaction was then heated to 323 K and stirred for 12 h. The reaction was cooled to room temperature and poured into ice water (~400 mL) with 30% H2O2 (3 mL). The excess H2O2 was added to the mixture until the observation of a permanent yellow color, which indicating the complete oxidation of graphite. The final product was obtained by centrifugation, followed by washed with deionized water, 30% diluted hydrochloric acid and absolute ethyl alcohol for many times and dried under vacuum 298 K.

Synthesis of NiP/rGO catalysts

All chemicals were commercial and used without further purification (Table 1). Ultrapure water was used as the reaction solvent.

In a typical experiment, GO (4 mg) was dispersed in ultrapure water (1.5 mL) kept in a 25 mL two-necked round bottom flask. Different amounts of NaH2PO2 solution (0.04 mmol, 0.08 mmol, 0.16 mmol, 0.24 mmol, 0.1 mol L1) were added into this flask, respectively. Ultrasonication was required to get a uniform dispersion. After adding NiCl2 (0.04 mmol, 0.1 mol L1) into the flask, the resulting mixture was reduced by aqueous solution containing NaBH4 (37.8 mg) with vigorous stirring at 298 K. After the reduction reaction were completely, the product was collected by centrifugation and washed twice in water, and dried by pump vacuum at room temperature for 4 h to give NiP/rGO catalysts as a black powder. Ni/rGO was synthesized in the same method without the addition of NaH2PO2.

Preparation of ammonia borane (AB)

Catalytic hydrolysis of AB

Sodium borohydride (NaBH4, 0.05 mol) and sodium ammonia sulfate ((NH4)2SO4, 0.1 mol) were added into 250 mL twonecked round-bottom flask with one neck connected to a condenser. Tetrahydrofuran (THF, 100 mL) was transferred into the flask with vigorously stirring at 313 K under nitrogen

The catalytic hydrolysis of AB by NiP/rGO catalysts were tested at 298 K. Various NiP/rGO catalysts with different phosphorus contents were added into a 25 mL two-necked round bottom flask, one neck was connected to a gas burette to monitor the volume of H2 released from AB hydrolysis, while the other neck

Experimental Chemicals and materials

Table 1 e The chemical reagents used and their manufacturers. Chemical reagents Nickel chloride hexahydrate Sodium hypophosphite Sodium borohydride Sodium ammonia sulfate Tetraphydrofuran Ethyl ether Potassium permanganate Hydrogen peroxide Phosphoric acid Sulfuric acid Graphite power

Chemical formula

Purity

NiCl2$6H2O NaH2PO2 NaBH4 (NH4)2SO4 THF C4H10O KMnO4 H2O2 H3PO4 H2SO4 C

99% >99% >98% >99% 99% 99.7% 99.5% 30% 85% 95% 99.85%

Manufacturer Sinopharm Chemical Reagent Aladdin Co., Ltd. Sinopharm Chemical Reagent Aladdin Co., Ltd. Sinopharm Chemical Reagent Sinopharm Chemical Reagent Shanghai Chemic Co., Ltd. Sinopharm Chemical Reagent Sinopharm Chemical Reagent Sinopharm Chemical Reagent Sinopharm Chemical Reagent

Co., Ltd. Co., Ltd. Co., Ltd. Co., Ltd. Co., Co., Co., Co.,

Ltd. Ltd. Ltd. Ltd.


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Fig. 1 e XPS spectra of (a) Ni 2p3/2 and (b) P 2p for Ni91P9/rGO. was used for the introduction of AB (1 mmol, 1 mol L1). A water bath was used to control the temperature of the reaction solution at 298 K. Also, the reaction were carried out at 303 K, 313 K and 323 K, while the amounts of NiP/rGO and AB were kept at the same molar ratio (catalyst/AB ¼ 0.04) to obtain the activation energy (Ea). The value of turnover frequency (TOF) was calculated by using following equation.

TOFinitial ¼

Patm VH2 =RT ; nNi t

where TOFinitial is initial turnover frequency, Patm is the atmospheric pressure, VH2 is the volume of the generated gas when the conversion reached 50%, R is the universal gas constant, T is room temperature (298 K), nNi is the mole amount of Ni, and t is the reaction time.

Cycle stability tests For the cycle stability tests, catalytic reactions were repeated 5 times by adding another equivalent of AB (1 mmol) into the mixture after the previous cycle.

Fig. 2 e (a) and (b) TEM images with different magnification, (c) EDX spectrum and (d) size distribution of Ni91P9/rGO.

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Fig. 3 e (a) Hydrogen generation from the hydrolysis of aqueous NH3BH3 (1 mmol) catalyzed by Ni94.9P5.1/rGO, Ni91P9/rGO, Ni89.1P10.9/rGO, Ni85.6P14.4/rGO and Ni/rGO. (b) Cycle stability test of Ni91P9/rGO for hydrogen generation from the hydrolysis of aqueous NH3BH3 (1 mmol) under an ambient atmosphere at 298 K (catalyst/NH3BH3 ¼ 0.04).

Physical characterizations Powder X-ray diffraction (XRD) patterns were measured by a Bruker D8-Advance X-ray diffractometer using Cu Ka radiation source (l ¼ 0.154178 nm) with a velocity of 8 /min. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was performed on IRIS Intrepid II XSP. X-ray photoelectron spectroscopy (XPS) measurement was performed with a Kratos XSAM 800 spectrophotometer. The morphologies and sizes of the samples were observed by Tecnai G20 U-Twin transmission electron microscope (TEM) equipped with an energy dispersive X-ray detector (EDX) at an acceleration voltage of 200 kV.

Results and discussion NiP/rGO with different phosphorus contents were successfully prepared by the in situ co-reduction method. In this reaction, NaH2PO2 was used as phosphorus source, NiCl2 was used as metal precursor and graphene oxide (GO) was used as support. The different contents of phosphorus were controlled by changing the amounts of NaH2PO2, which were further determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), as summarized in Table S1. As shown in Fig. S1a, from the X-ray photoelectron spectroscopy (XPS) results, the binding energy of GO at 284.6 eV, 286.6 eV and 288.5 eV can be assigned to sp2C, CeO/C]O and COO, respectively [37]. As shown in Fig. S1b, for Ni91P9/rGO, the binding energy at 284.6 eV, 285.5 eV and 288.6 eV were assigned to sp2C, CeO/C]O and COO, respectively. It was observed that the binding energy of CeO or C]O (285.5 eV) in Ni91P9/rGO was much smaller than that in GO (286.6 eV). And the peak intensity of COO in Ni91P9/rGO was much weaker than that in GO. These results indicated the successful reduction of GO to reduced graphene oxide (rGO). As shown in Fig. S2, from the Powder X-ray diffraction (XRD), the broad peak at 40e50 implied that the in situ prepared NiP possess an amorphous structure. Compared to the undoped Ni/rGO, NiP/rGO exhibited a little shift at 2-theta, which might be due to the incorporation of phosphorus [38,39].

The high-resolution X-ray photoelectron spectroscopy (XPS) spectra of Ni 2p3/2 and P 2p for NiP/rGO were shown in Fig. 1. In order to investigate the bonding state of Ni 2p3/2 and P 2p, deconvolution procedures were carried out. In Fig. 1a, the binding energy at 852.39 eV of Ni 2p3/2 spectrum for NiP/rGO was assigned to Nidþ, which exhibited a positive shift of 0.2 eV compared to metallic Ni peak at 852.2 eV (Fig. S3). The binding energy at 856.0 eV was attributed to Ni2þ derived from oxidization. The binding energy at 861.2 eV was assigned to the satellite peak originated from the shakeup of Ni2þ species. For the P 2p counterpart, the binding energy at 129.8 eV was assigned to Pd, which was negatively shifted from the elemental P (130.2 eV). The binding energy peak appeared at 133.6 eV was owing to the P5þ originated from oxidized P [40e42]. This result demonstrated that Ni in Ni91P9/rGO possess a partial positive charge (dþ), while P possess a partial negative charge (d), suggesting the electron transfer from metal center Ni (dþ) to pendant base P (d) in Ni91P9 nanoparticles. It has been reported that, during the AB hydrolysis reaction, P (d) can promote the formation of MH complex and Ni (dþ) can facilitate metal-catalyzed borohydride hydrolysis [43]. The microstructures of Ni91P9/rGO and Ni/rGO were investigated by Transmission electron microscope (TEM). Typically, the P-doping NiP nanoparticles in Ni91P9/rGO have

Table 2 e Comparison of activities and Ea of different catalysts for hydrogen generation from NH3BH3 hydrolysis. Catalyst Ag/C/Ni RGO/Pd NPs Ag@C@Co NiP/rGO [email protected]/C FeeNi alloy Ni NPs Ni/ZIF-8 Cu@CoNi/rGO Co/g-Al2O3 RuCu/Y-Al2O3

TOF (min1)

Ea (kJ mol1)

Ref.

5.32 6.25 8.93 13.3 e 10.7 8.8 14.2 15.46 2.3 8.2

38.91 51 e 34.7 41.5 e e e 58.41 62 52

[45] [46] [47] This work [48] [49] [50] [51] [52] [53] [54]


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Fig. 4 e (a) Hydrogen generation from the hydrolysis of aqueous NH3BH3 (1 mmol) catalyzed by Ni91P9/rGO at 298e323 K (catalyst/NH3BH3 ¼ 0.04). (b) Arrhenius plot: ln k versus 1/T.

an average diameter of approximately 4.7 nm (Fig. 2), while the undoped Ni nanoparticle in Ni/rGO possessing an average diameter of 5.1 nm (Fig. S4). The TEM of Ni91P9/rGO and Ni/rGO demonstrated that the addition of P could inhibit the aggregation and reduced the size of Ni91P9 nanoparticles. It has been reported that the incorporation of phosphorus can significantly decrease the crystallinity and particle size, probably due to the disconnection of crystallographic continuity of metal particles [44]. As shown in Fig. 2c, the energy dispersive X-ray spectrum (EDX) further illustrated the presence of Ni and P with an atomic ratio of 90.4:9.6, agreed well with the ICP-AES result (91:9) (Table S1). The catalytic activities of NiP/rGO with different phosphorus contents toward hydrolysis of AB were shown in Fig. 3a. The Ni/rGO without P doping exhibited a inferior catalytic activity, with the TOF value of 3.8 min1. Unexpectedly, Ni91P9/rGO exhibited a superior activity with the TOF value of 13.3 min1, which is much higher than those of Ni89.1P10.9/rGO (TOF ¼ 8.7 min1), Ni94.9P5.1/rGO (6.1 min1), Ni85.6P14.4/rGO (6.1 min1) and the other metal catalysts (As shown in Table 2). The enhanced catalytic performance of NiP/rGO for hydrolysis of AB might be attributed to the following advantages. First, owing to the strong electronic effect in Ni91P9/rGO, P doping can effectually modify the d-band electronic density of Ni, leading to a significantly improved catalytic activity for AB dehydrogenation. In addition, the amorphous structure of Ni91P9/rGO with more active sites also played an important role. Furthermore, the Ni91P9 NPs exhibited much smaller particle size distribution due to the P doping. The durability of Ni91P9/rGO up to fifth run for the catalytic hydrolysis of AB was also studied. As shown in Fig. 3b, during the recyclability test, the catalytic activity and conversion rate were still maintained well. In order to obtain the activation energy (Ea) of the hydrolysis of AB catalyzed by Ni91P9/rGO, dehydrogenation experiments were performed at temperatures ranging from 298 to 323 K, as shown in Fig. 4a. We used the slope of each linear part at different temperatures to calculate the reaction rate constant k. Arrhenius plot of ln k versus 1/T was shown in Fig. 4b. According to the Arrhenius equation, the activation energy (Ea) was calculated to be 34.7 kJ mol1, which is much

lower than those of other reported Ni-based catalysts and even some noble metal catalysts (Table 2).

Conclusions In summary, amorphous NiP nanoparticles supported on rGO have been successfully synthesized through a facile in situ coreduction method, and further used as highly efficient and robust catalysts for hydrogen generation from ammonia borane. Thanks to the efficient P doping and synergistic electronic effect between Ni and P, the resulted Ni91P9/rGO nanocatalyst exhibits superior catalytic activity toward AB hydrolysis with the TOF value of 13.3 min1. Moreover, the activation energy (Ea) of Ni91P9/rGO is measured to be 34.7 kJ mol1, further indicating its high catalytic activity and better kinetic performance. This facile synthetic method might open up new avenues for more applications of transition metal phosphides (TMPs).

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21571145), and Large-scale Instrument and Equipment Sharing Foundation of Wuhan University.

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.04.052.

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