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Fabrication of Pt–Co NPs supported on nanoporous graphene as high-efficient catalyst for hydrolytic dehydrogenation of ammonia borane
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Fabrication of PteCo NPs supported on nanoporous graphene as high-efficient catalyst for hydrolytic dehydrogenation of ammonia borane Dandan Ke a,b, Jin Wang a,b, Hongming Zhang a,b, Yuan Li b,**, Lu Zhang a,b, Xin Zhao c, Shumin Han a,b,* a
Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, PR China b State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, PR China c School of Materials and Metallurgy, Inner Mongolia University of Science and Technology, Baotou 014030, PR China
article info
abstract
Article history:
Nanoporous graphene (PG) supported PteCo bimetallic nanoparticles were prepared and
Received 28 May 2017
their catalytic activity in hydrogen generation from hydrolysis of NH3$BH3 solution were
Received in revised form
examined. The synthesized PteCo@PG with a loading amount of 30 wt% PteCo (atomic
19 September 2017
1 ratio 1:9) exhibited a superior TOF value of 461.17 molH2 min1 mol Pt and an activation
Accepted 23 September 2017
energy (Ea) value of 32.79 kJ mol1 for NH3$BH3 hydrolysis. This remarkably enhanced
Available online xxx
activity was ascribed to the charge interaction between PteCo NPs and PG support. The defects and holes on PG acting as the anchoring sites for PteCo NPs was beneficial for
Keywords:
achieving a uniform distribution and a decreased particle size for the NPs. The PteCo@PG
Nanoporous graphene
catalysts also showed a well-established reusability, with 81.2% of their initial catalytic
PteCo bimetallic catalysts
activity after five runs of reactions, demonstrating that they had high durability.
Hydrolytic dehydrogenation
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Ammonia borane
Introduction Secure storage and effective release of hydrogen are very important in the application of hydrogen energy for solving ever-growing energy crisis and achieving practical applications of hydrogen powered vehicles and electronic devices [1]. Finding effective hydrogen generation/storage systems that can hold sufficient hydrogen in terms of gravimetric and volumetric densities and have suitable thermodynamic and
kinetic properties is one of the toughest challenges. Tremendous efforts have been devoted to research and development on hydrogen storage materials such as sorbent materials or hydrides. Among the chemical hydrides, ammonia borane (NH3$BH3) has emerged as an suitable candidate, with merits of its high hydrogen storage gravimetric efficiency (with a theoretical value of 19.6 wt%) and well-behaved stability under ambient conditions [2,3]. Hydrogen stored in ammonia borane can be released through catalytic dehydrogenation according to Eq. (1).
* Corresponding author. Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, PR China. ** Corresponding author. E-mail addresses: [email protected] (Y. Li), [email protected] (S. Han). https://doi.org/10.1016/j.ijhydene.2017.09.121 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Ke D, et al., Fabrication of PteCo NPs supported on nanoporous graphene as high-efficient catalyst for hydrolytic dehydrogenation of ammonia borane, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.121
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catalyst
NH3 BH3 þ 2H2 O !NH4 BO2 þ 3H2 [
(1)
In the past, noble metal-based catalysts were the mainstream of heterogeneous catalysts for the advantages of excellent catalytic activity and well reusability on the hydrolysis of NH3$BH3, but the high price and scarce resources of the noble metal obstructed their practical applications [4e6]. Recent efforts have been devoted designing new catalysts with minimum usage of Pt which include concepts like alloying metals [7], Pt monolayer approach [8] and dealloying method [9]. Partially substituting Pt with a less expensive nonnoble metal as promoters, such as Ni, Cu, and Co [10,11], has attracted considerable attention. There are many studies on successful synthesis of PteM bimetallic NPs with controllable size, shape, and composition as well as high-efficiency catalytic performance [12,13]. Wang's group [13] studied the catalytic activities of PteM NPs in hydrolytic dehydrogenation of AB and revealed that PteNi NPs with a ratio of 4:1 show the best catalytic activity. Abundant investigations have revealed that the catalytic performance for the hydrolysis of NH3$BH3 is highly associated with the dispersity and size of active metal nanoparticles (NPs). However, in the repeatedly process of catalytic, PteM NPs are readily appearing agglomeration phenomenon, leading to a decrease on catalytic activity. So in order to acquire high activity in hydrolytic dehydrogenation of NH3$BH3, substantial effort has been devoted to explore highefficiency catalysis system involving supported metal catalysts [14,15]. Recently, graphene, a single-layer of sp2 carbon lattices, intrinsically holding fascinating properties such as high specific surface area and outstanding charge carrier mobility, could be an ideal substrate for anchoring metal nanoparticles (NPs) for various practical applications such as electronics, optics and biotechnology, as well as a variety of catalysis [16]. Nowadays, various derivative of graphene sheets with novel morphological transformation, such as 3D graphene oxide hydrogels, heteroatoms (e.g. N, S, P and B) doped graphene [17,18], organic polymers modified graphene composite [19], is attracting growing interest. Thereinto, defect-functionalized porous graphene exhibits excellent performances, owing to the defects and pores in the pristine graphene substrate benefits in facilitating the mass transfer and charge interaction, enhancing the dispersion and stability of metal NPs, thereby improving the catalytic activity. However, the successful preparation of porous graphene in large scale with low cost is of challenging. Kumar's research group produced a three-dimensionally nanohole-structured and palladium-embedded 3D porous graphene catalyst (3D Pd-E-PG) via microwave irradiation method for ultrahigh hydrogen storage and CO oxidation multifunctionalities [20]. Zhou and co-workers [21] have successfully reported a general and scalable synthesis approach for nano-scaled (1e50 nm) pores porous graphene through the carbothermal reaction between graphene and metal oxide nanoparticles produced from oxometalates (OM) or polyoxometalates (POM). Moreover, exploring the fabrication of embedded metal NPs in porous graphene as well as the study of the synergistic effect between them are essential for catalytic investigation.
Hence, in this work, we investigated catalytic characteristics of PteCo bimetallic NPs embedded in nanoporous graphene (PteCo@PG). Microscopic detections and spectroscopic studies were undertaken to understand the interaction between the porous graphene substrate and the embedded bimetallic NPs. We also studied the catalytic dehydrogenation performance from an aqueous NH3$BH3 solution involving the hydrolysis kinetics parameters such as supporting feature and temperature, as well as recycling performance in detail.
Experimental Nanoporous graphene synthesis Nanoporous graphene (PG) was prepared by a carbothermal etching method of graphene oxides and metal oxide nanoparticles and subsequent removal of etcher [21]. Typically, graphene oxide (GO) was prepared by chemical exfoliation of graphitic material according to a modified Hummers method [22]. GO aqueous dispersion (4 mg/mL, 100 mL) was mixed with sodium molybdate (Na2MoO4$2H2O, 2.26 mmol) and ultrasonicated for 1 h, then stirred overnight with a magnetic stirrer. The homogeneous mixture of GOeNa2MoO4 was freeze-dried at 48 C for 2e3 days, and then heating treated at 650 C for 4 h with heating rate of 5 C min1 under 600 C and 1 C min1 above 600 C. PG was obtained after immersing the black product, denoted as GOM, in aqueous acid solution (HCl, 1 M) for more than a week. Finally, PG samples was washed with deionized H2O and ethanol, and dried under vacuum freeze dehydration.
Synthesis of PteCo@PG NPs To deposit metal elements (M ¼ Pt, Co and PteCo) on PG, various molar ratios of H2PtCl6 and CoCl2 as the metal precursors have been used to prepare the final products donated as PtxCo1x@PG (x ¼ 0.1, 0.2, 0.3, 0.4 and 0.5). To synthesize the PtxCo1x@PG, 6.5 mg as-synthesized PG was mixed with 10 mL ethanolewater solvent (volume ratio of 1:3) and sonicated for 30 min. 1 mL H2PtCl6 (0.005 M) and corresponded volume of CoCl2 (0.045 M) were well mixed with the PG suspension for 2 h under nitrogen atmosphere. Then, 5-fold molar excess of freshly prepared 0.5 M NaBH4 was added and the solution immediately changed into dark brown. This solution was further stirred under nitrogen atmosphere for about 20 min until no hydrogen ceased to ensure all NaBH4 had hydrolyzed. Upon completion, the product in a suspension liquid was washed with deionized water and ethanol, and dried under vacuum freeze dehydration.
Catalytic activity measurement All hydrolytic dehydrogenation tests were monitored using a classic inverted water displacement method. 6 mL freshly prepared NH3$BH3 aqueous solution (containing 1.5 mmol of AB) was added into the as-synthesized PteCo @PG NPs while stirring at room temperature and elevated temperatures controlled by a thermostatic water bath equipped with rotary reciprocating oscillation. The volume of hydrogen was
Please cite this article in press as: Ke D, et al., Fabrication of PteCo NPs supported on nanoporous graphene as high-efficient catalyst for hydrolytic dehydrogenation of ammonia borane, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.121
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monitored by recording the displacement of water level in an inverted and graduated water-filled burette. The turnover frequency (TOF, molH2 min1 mol1 Pt ) of AB hydrolysis was estimated from the linear portion of the volume/time plot. After reaction, the catalyst was separated by an external magnet and dried overnight under vacuum freeze dehydration. 6 mL of deionized water was added to the flask and 1.5 mmol NH3$BH3 was added with shaking. Then, the repeated dehydrogenation reaction was performed 5 times in the same way under ambient temperature to measure the recyclability of the Pt0.1Co0.9@PG catalyst.
Structural characterization Specimen for transmission electron microscopy (TEM) test was prepared by mechanical lapping under alcohol then processed by ultrasounding for 30 min. TEM was performed on a JEM-2010 transmission electronmicroscopy. Morphology of the samples was obtained on Hitachi S-4800 scanning electron microscopy (SEM) with energy-dispersive X-ray (EDX) operating at 15.0 kV to determine the chemical composition of the sample. X-ray diffraction (XRD) analyses were carried out using a Rigaku D/Max-2500/PC X-ray diffractometer with Cu Ka radiation (l ¼ 1.544 A) at a scanning rate of 4 min1 in 2q range from 10 to 80 . Powder X-ray photoelectron spectrometry (XPS) measurements were performed on a PHI5000Versa Probe X-ray photoelectron spectrometer with monochromatic Al Ka radiation.
Results and discussion Microstructure of PteCo@PG catalyst Fig. 1 shows images for GO at different stages. Compared to the pristine GO with a smooth surface (Fig. 1(a)), TEM image of GO-Na2MoO4 calcinated at 650 C (Fig. 1(b)) clearly shows that the carbothermal products are homogeneously attached on graphene sheet. After acid treatment to get rid of metal oxide, PG (Fig. 1(c)) maintains lamellar graphene structure with abundant nano-scaled pores of 10e30 nm across the entire plane. It indicates that the carbothermal products were reacted with GO matrix successfully and existed as insertion for the formation of nanopores. The bared PteCo NPs are irregular spherical shape with clear particle aggregation and the particle diameter is about
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20 nm (Fig. 2(a)). GO assisted PteCo NPs deposition reveals an improved control to the particle morphology with a slight aggregation (Fig. 2(b)). Furthermore, PteCo particles on PG exhibit an excellent distribution around the matrix nanopores (Fig. 2(c)). It also shows that the PteCo sizes for PteCo@rGO and PteCo@PG are different on uniformity. On the GO surface, the size of PteCo particle is from 5 nm to 15 nm, while that of the NPs on PG is uniformly about 8 nm. This is ascribed to the edge formed by nanopores on PG provide abundant attachment sites which are beneficial to the nucleation of PteCo particles during coprecipitation process. To clarify the formation mechanism of PteCo@PG catalysts, the procedure for fabricating PteCo NPs deposited on nanoporous graphene (PG) sheets is shown in Scheme 1. It contains two important steps, namely, the synthesis of nanoporous graphene sheets, and subsequent deposition of bimetallic PteCo NPs via a facile chemical reduction method. As depicted in the TEM images Fig. 1(aec), the pristine graphene oxide (GO) sheets, present nearly transparent and smooth plane. Nanoporous graphene sheets are fabricated via carbothermal reaction by using Na2MoO4 as etching agent. It is worth noting that in the formation of nanoporous graphene sheets, Na2MoO4 as etching agent after calcination treatment with pristine GO changes into metal oxides adsorbed in the surface. The adjacent carbon atoms of GO is oxidized into CO2. The porous graphene is formed after acid treatment to get rid of metal oxide. The successful deposition of bimetallic PteCo NPs on the PG support is further confirmed by the powder X-ray diffraction (XRD), Raman spectra and X-ray photoelectron spectroscopy, shown in Fig. 3. A broad peak centered at 2q value of 23.62 which characterizes its layer spacing is observed for PG, and the diffraction peak at 10 of the characteristic peak for GO sheet disappears. The changes suggest that the transformation from GO to PG accompanied by the removal of various oxygen-containing functional segments on the GO surface [23]. The XRD peaks of the as-synthesized PteCo@PG catalyst centered at 40.1 , 46.7 and 68.2 are clearly strong, which exhibit the typical Pt face-centered cubic features (PDF # 04-0904), whereas the Co diffraction peaks are invisible. Absence of the Bragg peaks in the XRD pattern is the identification of an amorphous nature of Co in the PteCo bimetallic components [24]. It is worth to mention that the diffraction peak of pure PG at 23.62 moves to 26.34 after loading, indicating that the laying spacing of PG matrix decreased from 3.7662 A to 3.4081 A by the loading of PteCo nanoparticles.
Fig. 1 e TEM images of (a) pristine graphene oxide sheets, (b) GO-Na2MoO4 calcinated at 650 C, (c) nanoporous graphene (PG) sheets. Please cite this article in press as: Ke D, et al., Fabrication of PteCo NPs supported on nanoporous graphene as high-efficient catalyst for hydrolytic dehydrogenation of ammonia borane, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.121
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Fig. 2 e TEM images of (a) PteCo NPs, (b) PteCo@rGO NPs and (c) PteCo@PG NPs.
Scheme 1 e Schematic of PteCo@PG catalysts preparation.
Raman spectra for GO, PG and the composites are shown in Fig. 3 (b). Each spectrum shows two obvious bands centered at 1596 and 1322 cm1, assigned to the G and D bands of the carbon components, respectively. The G band corresponds to the in-plane vibration of sp2 carbon atoms in a 3D hexagonal lattice, while the D band corresponds to the vibrations of sp3 carbon atoms of disordered graphite [25]. On the other hand, the intensity ratio of G and D bands is an important parameter of defect density in the characterization of carbon materials [26]. In the present case, the results indicate that the value of ID/IG changes from 0.77 to 1.16 after the carbothermal reaction for the transformation from GO to PG, which implies the defects increased significantly during the preparation of PG. Also, the substrate defects are further increased after PteCo NPs deposited on the PG scaffold as the value of ID/IG for the PteCo@PG composite is increased to 1.31. It has been verified that the defects on the supporting materials contribute more anchors for metal nanoparticles and have a strong correlation on improving the catalytic activity [19]. Fig. 3(c) shows the XPS survey spectra of GO, PG and PteCo@PG. The high-resolution C1s XPS spectrum (Fig. 3(d)) of as-prepared GO are deconvoluted to five types of carbon bond centered at 284.58, 285.6, 286.7, 287.1, and 288.6 eV are observed, corresponding to sp2C, sp3C, eCeO, eC]O, and e COO groups, respectively [14]. However, after carbothermal reaction, the majority of oxygen functional groups and the sp3 carbon have been removed and the sp2 C bonds with 284.5 eV
are dominant, indicating most of the graphene oxides are reduced and the conjugated graphene networks are restored, which also agrees well with the Raman results. Meanwhile, compared with the bared PG, the C1s XPS spectrum of PteCo@PG centered at 284.7 eV indicates that the structure of nanoporous graphene is unchanged. The positive shift results in the charge interactions between PteCo NPs and PG support (Fig. 3 (e)), which defined as a synergistic effect, have been proved as the principally element to elevate the catalytic performance of the hybrid nanocatalyst to a great extent [27]. Furthermore, XPS studies showed that different Pt and Co states existed in the PteCo@PG catalyst. As illustrated in Fig. 3(e), the most intense doublet of Pt4f spectrum with binding energies of 70.87 and 74.03 eV represents the zerovalent metallic state, developing a chemical shift of 0.22 eV compared to its standard values (71.0 eV). The Co2P3/2 spectra with three binding energy (BE) of 778.6 eV, 781.6 eV and 786.2 eV are observed corresponding to Co metal, Co(II) and Co(III), respectively. The XPS signal of oxidized Co results from oxidation during sample handling for XPS measurements and preservation process [28]. By comparing the BE of metal Co (778.3 eV), we observe a positive shift of 0.3 eV. The increase in binding energy of Co could be attributed to the fact that metal Co acts as an electron donor and contributes partial electrons to Pt, making Co electron-deficient and Pt electron-enriched in their corresponding alloy NPs. The charge transfer from the more electropositive Co elements to Pt implies a downshift of
Please cite this article in press as: Ke D, et al., Fabrication of PteCo NPs supported on nanoporous graphene as high-efficient catalyst for hydrolytic dehydrogenation of ammonia borane, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.121
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Fig. 3 e (a) XRD patterns of GO, PG, PteCo NPs, PteCo@PG NPs, (b) Raman spectra of GO, PG, PteCo@PG NPs, (c) X-ray photoelectron survey spectra (XPS) of GO, PG, PteCo@PG NPs, (d) XPS C1s deconvolution of GO, (e) XPS C1s deconvolution of PG, PteCo@PG NPs, and (f) XPS Pt4f and Co2p deconvolution of PteCo@PG NPs. d-band center of Pt, which is strongly correlated with the adsorption energy, thus affecting the catalytic performance in the catalysis reaction [29].
Catalytic activity of PteCo@PG catalyst on NH3·BH3 hydrolysis Ammonia borane (1.5 mmol, 6 mL) can be catalytically hydrolyzed by freshly prepared nanoporous graphene supported pure Pt NPs, pure Co NPs and PteCo NPs with different composition, respectively. The comparison experiments are carried out under the same conditions at room temperature, which is an ideally mild condition for practical usage. Fig. 4(a) shows the plots of the amount of hydrogen generated during
the hydrolytic dehydrogenation process with the freshly prepared PtxCo1x@PG NPs. Clearly, pure PG shows no catalytic activity for the hydrolysis of AB. PteCo@PG NPs show a much higher H2-release rate than pure Co@PG NPs. The complete hydrolysis of AB finished in less than 3 min for Pt-based catalysts, 9.4 min for Co@PG NPs. Turnover frequency (TOF, molH2 min1 mol1 Pt ) value estimated from the linear portion of the H2 volume/time plot based on total metal and noble metal Pt content are listed in Fig. 4(a), respectively. TOF value is a valid and economic criterion for catalytic activity. Though pure Pt@PG catalyst finished the whole hydrolysis reaction in almost 1 min, suggesting Pt is a more effective element for the hydrolysis of AB, the TOF value based on noble metal Pt content is sharply decreased as compared with PteCo bimetallic
Please cite this article in press as: Ke D, et al., Fabrication of PteCo NPs supported on nanoporous graphene as high-efficient catalyst for hydrolytic dehydrogenation of ammonia borane, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.121
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Fig. 4 e (a) Hydrolysis of aqueous NH3·BH3 solution under ambient atmosphere catalyzed by pure Pt@PG NPs, pure Co@PG NPs, and PteCo@PGNPs with different ratios; (b) Hydrolysis of aqueous NH3·BH3 solution catalyzed by Pt0.1Co0.9 NPs, Pt0.1Co0.9@GO NPs and Pt0.1Co0.9@PG.
catalysts. The catalysis mechanism of PteCo bimetallic NPs could be ascribed to the synergistic effects of Co and Pt caused by charge interactions between PteCo NPs and PG support and the decreased particle size which providing abundance active site [13]. The PteCo synergistic effect could also rely on the atomic ratio of these two elements. It has been reported that according to the volcano shape plotted as a function of the strength of hydrogen-metal bond, a high-efficiency catalyst should have median binding energies (or free energies of adsorption) for reactive intermediates [30,31]. If the bonding energy between metal atoms is too strong, they will restrain the reactivity of reactants and poison the catalyst. Otherwise, if the bonding energy is too weak, the adsorption quantity will be too little, thus lower the catalytic reaction rate [8,32]. Hence, in heterogeneous catalysis for PteCo bimetallic NPs, there is an optimal ratio of Pt and Co to show the highest catalytic activity. The Pt:Co ratio is optimized to achieve the best performance with Pt0.1Co0.9@PG (Pt:Co ¼ 1:9). The optimized Pt0.1Co0.9@PG exhibits markedly high catalytic activity to release a stoichiometric amount of H2 in the hydrolysis of aqueous NH3$BH3 (H2/NH3$BH3 ¼ 3.0) in 3 min with a turnover frequency (TOF) value of 461.17 molH2 min1 mol1 Pt . The excellent performance of Pt0.1Co0.9@PG is evident by comparing its TOF value to those previously reported, as shown in Fig. 5(b).
The supporting effect of nanoporous graphene is clearly demonstrated as compared with bared Pt0.1Co0.9 catalyst and reduced graphene oxide supported Pt0.1Co0.9 catalyst, as shown in Fig. 4 (b). Pt0.1Co0.9 NPs deposited on PG samples show much higher TOF value in sharp contrast to the values of Pt0.1Co0.9@rGO (4 min, 312.46 molH2 min1 mol1 Pt ) and the sample (6 min, 179.66 unsupported Pt0.1Co0.9 molH2 min1 mol1 Pt ). The state-of-the art investigations indicate the enhanced catalytic activity of graphene supported metal NPs should result from the interfacial interaction between metal NPs and graphene materials [33]. The nanoporous graphene sheets provide more edges associated with the existence of holes and contribute more anchors for stabilizing PteCo NPs with homogeneous particle size. These enhancements in the synthesis of nanoporous graphene sheets supported transition metal catalyst will extent the potential applications on hydrogen storage materials, environmental catalysis, electrocatalysis and biotechnology. Fig. 5(a) further illustrates the influence of PG loading percentage on the catalytic performance of Pt0.1Co0.9@PG catalyst for hydrolytic dehydrogenation of NH3$BH3 solution. The effect of PG content on the catalytic activity of the catalysts can be clearly seen in a trend that with increasing PG content, the H2 generation rates gradually increase with more PG loadings up to the optimum value of 30 wt %, then
Fig. 5 e (a) Hydrogen evolution from aqueous NH3·BH3 solution catalyzed by Pt0.1Co0.9@PG-x% (x%: weight percentage of PG support, x ¼ 0e70); (b) A comparison of our Pt0.1Co0.9@PG catalyst with the reported noble metal-based monometallic, bimetallic catalysts in terms of TOF and Ea value in the catalytic dehydrogenation of NH3·BH3. Please cite this article in press as: Ke D, et al., Fabrication of PteCo NPs supported on nanoporous graphene as high-efficient catalyst for hydrolytic dehydrogenation of ammonia borane, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.121
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decrease as the PG loadings further increase. This may be due to that too little PG support is adverse to the dispersion of metal NPs. Agglomeration of excess PteCo NPs on the PG support hinders some of the active site during the hydrolysis reaction. Meanwhile, excessive PG loadings will reduce the content of active metal component and limit the available active sites at the interface of metal and support. As a result, after optimizing the various parameters for the dehydrogenation of NH3$BH3, the best catalyst PteCo@PG with a metal loading of 30 wt% and the atomic ratio of 0.1:0.9 achieved a remarkable TOF value of 461.17 molH2 min1 mol1 Pt . Compared with other ever reported noble metal-based catalysts and Co catalysts (Fig. 5(b)) [18,34e40], the Pt0.1Co0.9@PG catalyst is among the most active catalyst, superior to the majority of noble metal-based catalyst, such as Pd@Co/graphene catalyst [39] and Ru (0)@MWCNT catalyst [34]. We believe that this excellent performance arises from the contribution of the nanoporous graphene sheets which gives PteCo NPs better dispersion, and thus a higher availability of reactive sites, as well as facilitates the charge transfer interaction at the metal-porous graphene interface during the hydrolysis reaction. Fig. 6(a) shows the effect of temperature on the hydrolysis of aqueous NH3$BH3 solution. The values of the rate constant k at different temperatures are calculated from the slope of the linear part of each plot. Under our experimental conditions, k is constant for a given temperature, implying zero order kinetics for the NH3$BH3 hydrolysis reaction. According to the Arrhenius plot of ln(k) versus 1/T (Fig. 6(b)), the activation energy (Ea) value for the hydrolysis is calculated to be 32.79 kJ mol1. Compared with what has been reported on the hydrolytic performance of various Co-, Pt-, and other noble
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metal-based catalysts (Fig. 5(b)), the favorable activation energy value obtained in this work is again an evidence of the auxo-action by Pt0.1Co0.9@PG catalyst to enhance the catalytic activity. We should note that in Fig. 5(b) the completion time for noble metal-based catalysts varies in a wide range due to the nanoparticle size and supporting materials effects; (2) the data obtained from Ag@Co/graphene catalyst [35] is reduced by MeAB and AB and the dehydrogenation of AB is catalyzed by in-situ hydrolysis. As a crucial issue in the practical application of heterogeneous catalyst, the recycle stability of Pt0.1Co0.9@PG catalyst up to fifth run for hydrolysis of AB is tested and shown in Fig. 6(cee). The as-synthesized Pt0.1Co0.9@PG NPs are magnetic and thus bring the advantage of easy separation from the reaction solution by an external magnet. When H2 generation was complete, another equivalent (1.5 mmol) of AB was added to the reaction system and the released gas was once again measured. As shown in Fig. 6(c), there is only a slight decrease in catalytic activity even after five runs of reactions retaining 81.2% of their initial catalytic activity, indicating a wellestablished reusability for practical applications. The slight activity drop is caused by the increase in concentration of metaborate and the viscosity of the solution during the NH3$BH3 hydrolysis [41,42]. Fig. 6(e) shows the TEM image of Pt0.1Co0.9@PG NPs after the fifth run durability test. Clearly, the nanoporous structure and deposited Pt0.1Co0.9 NPs still maintain in good morphologies, and no obvious aggregation and dissolution are observed, which further confirm the good stability of Pt0.1Co0.9 NPs anchored on nanoporous graphene. The results from both hydrolysis activity and stability studies confirm that PteCo@PG is a desirable catalyst for dehydrogenation from NH3$BH3.
Fig. 6 e (a) Plots of time versus H2 generation volume at various temperatures from 293 K to 313 K; (b) Arrhenius plots and TOF values of NH3·BH3 dehydrogenation over 293 Ke313 K; (c) Photographs of the Pt0.1Co0.9@PG dispersions before and after magnetic separation; (d) Hydrolysis of aqueous NH3·BH3 solution catalyzed by Pt0.1Co0.9@PG from the 1st to 5th cycle; (e) TEM image after five cycles. Please cite this article in press as: Ke D, et al., Fabrication of PteCo NPs supported on nanoporous graphene as high-efficient catalyst for hydrolytic dehydrogenation of ammonia borane, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.121
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Conclusions In conclusion, novel PteCo NPs embedded in nanoporous graphene sheets as a high-efficiency catalyst for hydrolytic dehydrogenation of ammonia borane was reported. Nanoporous graphene sheets were fabricated via carbothermal metal oxide etching method, and then PteCo NPs were uniformly embedded in the planes and hole defects. The nanoporous structures not only acted as the anchoring site for stabilizing and dispersing the PteCo NPs, but also yielded a synergistic effect between nanoporous graphene and metal NPs on enhancing the charge transfer and improving catalytic activity and stability. PteCo@PG catalyst exhibited an initial turnover frequency (TOF) of 461.17 molH2 min1 mol1 Pt and retained 81.2% of their initial activity with a complete release of hydrogen in five runs for hydrolytic dehydrogenation of AB, which was superior to most noble metal-based catalysts. Activation energy for the hydrolysis of AB was determined as 32.17 kJ mol1 from the evaluation of kinetic data at different temperatures. Facile preparation and high catalytic performances revealed that PteCo@PG nanohybrids was a promising candidate in developing highly efficient, portable hydrogen generation systems using ammonia borane.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NOs. 21303157 and 51571173), Scientific Research Projects in Colleges and Universities in Hebei Province (QN2016002), and the Innovation Fund for the Graduate Students of Hebei Province (2016SJBS018).
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Fabrication of PteCo NPs supported on nanoporous graphene as high-efficient catalyst for hydrolytic dehydrogenation of ammonia borane Dandan Ke a,b, Jin Wang a,b, Hongming Zhang a,b, Yuan Li b,**, Lu Zhang a,b, Xin Zhao c, Shumin Han a,b,* a
Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, PR China b State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, PR China c School of Materials and Metallurgy, Inner Mongolia University of Science and Technology, Baotou 014030, PR China
article info
abstract
Article history:
Nanoporous graphene (PG) supported PteCo bimetallic nanoparticles were prepared and
Received 28 May 2017
their catalytic activity in hydrogen generation from hydrolysis of NH3$BH3 solution were
Received in revised form
examined. The synthesized PteCo@PG with a loading amount of 30 wt% PteCo (atomic
19 September 2017
1 ratio 1:9) exhibited a superior TOF value of 461.17 molH2 min1 mol Pt and an activation
Accepted 23 September 2017
energy (Ea) value of 32.79 kJ mol1 for NH3$BH3 hydrolysis. This remarkably enhanced
Available online xxx
activity was ascribed to the charge interaction between PteCo NPs and PG support. The defects and holes on PG acting as the anchoring sites for PteCo NPs was beneficial for
Keywords:
achieving a uniform distribution and a decreased particle size for the NPs. The PteCo@PG
Nanoporous graphene
catalysts also showed a well-established reusability, with 81.2% of their initial catalytic
PteCo bimetallic catalysts
activity after five runs of reactions, demonstrating that they had high durability.
Hydrolytic dehydrogenation
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Ammonia borane
Introduction Secure storage and effective release of hydrogen are very important in the application of hydrogen energy for solving ever-growing energy crisis and achieving practical applications of hydrogen powered vehicles and electronic devices [1]. Finding effective hydrogen generation/storage systems that can hold sufficient hydrogen in terms of gravimetric and volumetric densities and have suitable thermodynamic and
kinetic properties is one of the toughest challenges. Tremendous efforts have been devoted to research and development on hydrogen storage materials such as sorbent materials or hydrides. Among the chemical hydrides, ammonia borane (NH3$BH3) has emerged as an suitable candidate, with merits of its high hydrogen storage gravimetric efficiency (with a theoretical value of 19.6 wt%) and well-behaved stability under ambient conditions [2,3]. Hydrogen stored in ammonia borane can be released through catalytic dehydrogenation according to Eq. (1).
* Corresponding author. Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, PR China. ** Corresponding author. E-mail addresses: [email protected] (Y. Li), [email protected] (S. Han). https://doi.org/10.1016/j.ijhydene.2017.09.121 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Ke D, et al., Fabrication of PteCo NPs supported on nanoporous graphene as high-efficient catalyst for hydrolytic dehydrogenation of ammonia borane, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.121
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catalyst
NH3 BH3 þ 2H2 O !NH4 BO2 þ 3H2 [
(1)
In the past, noble metal-based catalysts were the mainstream of heterogeneous catalysts for the advantages of excellent catalytic activity and well reusability on the hydrolysis of NH3$BH3, but the high price and scarce resources of the noble metal obstructed their practical applications [4e6]. Recent efforts have been devoted designing new catalysts with minimum usage of Pt which include concepts like alloying metals [7], Pt monolayer approach [8] and dealloying method [9]. Partially substituting Pt with a less expensive nonnoble metal as promoters, such as Ni, Cu, and Co [10,11], has attracted considerable attention. There are many studies on successful synthesis of PteM bimetallic NPs with controllable size, shape, and composition as well as high-efficiency catalytic performance [12,13]. Wang's group [13] studied the catalytic activities of PteM NPs in hydrolytic dehydrogenation of AB and revealed that PteNi NPs with a ratio of 4:1 show the best catalytic activity. Abundant investigations have revealed that the catalytic performance for the hydrolysis of NH3$BH3 is highly associated with the dispersity and size of active metal nanoparticles (NPs). However, in the repeatedly process of catalytic, PteM NPs are readily appearing agglomeration phenomenon, leading to a decrease on catalytic activity. So in order to acquire high activity in hydrolytic dehydrogenation of NH3$BH3, substantial effort has been devoted to explore highefficiency catalysis system involving supported metal catalysts [14,15]. Recently, graphene, a single-layer of sp2 carbon lattices, intrinsically holding fascinating properties such as high specific surface area and outstanding charge carrier mobility, could be an ideal substrate for anchoring metal nanoparticles (NPs) for various practical applications such as electronics, optics and biotechnology, as well as a variety of catalysis [16]. Nowadays, various derivative of graphene sheets with novel morphological transformation, such as 3D graphene oxide hydrogels, heteroatoms (e.g. N, S, P and B) doped graphene [17,18], organic polymers modified graphene composite [19], is attracting growing interest. Thereinto, defect-functionalized porous graphene exhibits excellent performances, owing to the defects and pores in the pristine graphene substrate benefits in facilitating the mass transfer and charge interaction, enhancing the dispersion and stability of metal NPs, thereby improving the catalytic activity. However, the successful preparation of porous graphene in large scale with low cost is of challenging. Kumar's research group produced a three-dimensionally nanohole-structured and palladium-embedded 3D porous graphene catalyst (3D Pd-E-PG) via microwave irradiation method for ultrahigh hydrogen storage and CO oxidation multifunctionalities [20]. Zhou and co-workers [21] have successfully reported a general and scalable synthesis approach for nano-scaled (1e50 nm) pores porous graphene through the carbothermal reaction between graphene and metal oxide nanoparticles produced from oxometalates (OM) or polyoxometalates (POM). Moreover, exploring the fabrication of embedded metal NPs in porous graphene as well as the study of the synergistic effect between them are essential for catalytic investigation.
Hence, in this work, we investigated catalytic characteristics of PteCo bimetallic NPs embedded in nanoporous graphene (PteCo@PG). Microscopic detections and spectroscopic studies were undertaken to understand the interaction between the porous graphene substrate and the embedded bimetallic NPs. We also studied the catalytic dehydrogenation performance from an aqueous NH3$BH3 solution involving the hydrolysis kinetics parameters such as supporting feature and temperature, as well as recycling performance in detail.
Experimental Nanoporous graphene synthesis Nanoporous graphene (PG) was prepared by a carbothermal etching method of graphene oxides and metal oxide nanoparticles and subsequent removal of etcher [21]. Typically, graphene oxide (GO) was prepared by chemical exfoliation of graphitic material according to a modified Hummers method [22]. GO aqueous dispersion (4 mg/mL, 100 mL) was mixed with sodium molybdate (Na2MoO4$2H2O, 2.26 mmol) and ultrasonicated for 1 h, then stirred overnight with a magnetic stirrer. The homogeneous mixture of GOeNa2MoO4 was freeze-dried at 48 C for 2e3 days, and then heating treated at 650 C for 4 h with heating rate of 5 C min1 under 600 C and 1 C min1 above 600 C. PG was obtained after immersing the black product, denoted as GOM, in aqueous acid solution (HCl, 1 M) for more than a week. Finally, PG samples was washed with deionized H2O and ethanol, and dried under vacuum freeze dehydration.
Synthesis of PteCo@PG NPs To deposit metal elements (M ¼ Pt, Co and PteCo) on PG, various molar ratios of H2PtCl6 and CoCl2 as the metal precursors have been used to prepare the final products donated as PtxCo1x@PG (x ¼ 0.1, 0.2, 0.3, 0.4 and 0.5). To synthesize the PtxCo1x@PG, 6.5 mg as-synthesized PG was mixed with 10 mL ethanolewater solvent (volume ratio of 1:3) and sonicated for 30 min. 1 mL H2PtCl6 (0.005 M) and corresponded volume of CoCl2 (0.045 M) were well mixed with the PG suspension for 2 h under nitrogen atmosphere. Then, 5-fold molar excess of freshly prepared 0.5 M NaBH4 was added and the solution immediately changed into dark brown. This solution was further stirred under nitrogen atmosphere for about 20 min until no hydrogen ceased to ensure all NaBH4 had hydrolyzed. Upon completion, the product in a suspension liquid was washed with deionized water and ethanol, and dried under vacuum freeze dehydration.
Catalytic activity measurement All hydrolytic dehydrogenation tests were monitored using a classic inverted water displacement method. 6 mL freshly prepared NH3$BH3 aqueous solution (containing 1.5 mmol of AB) was added into the as-synthesized PteCo @PG NPs while stirring at room temperature and elevated temperatures controlled by a thermostatic water bath equipped with rotary reciprocating oscillation. The volume of hydrogen was
Please cite this article in press as: Ke D, et al., Fabrication of PteCo NPs supported on nanoporous graphene as high-efficient catalyst for hydrolytic dehydrogenation of ammonia borane, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.121
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monitored by recording the displacement of water level in an inverted and graduated water-filled burette. The turnover frequency (TOF, molH2 min1 mol1 Pt ) of AB hydrolysis was estimated from the linear portion of the volume/time plot. After reaction, the catalyst was separated by an external magnet and dried overnight under vacuum freeze dehydration. 6 mL of deionized water was added to the flask and 1.5 mmol NH3$BH3 was added with shaking. Then, the repeated dehydrogenation reaction was performed 5 times in the same way under ambient temperature to measure the recyclability of the Pt0.1Co0.9@PG catalyst.
Structural characterization Specimen for transmission electron microscopy (TEM) test was prepared by mechanical lapping under alcohol then processed by ultrasounding for 30 min. TEM was performed on a JEM-2010 transmission electronmicroscopy. Morphology of the samples was obtained on Hitachi S-4800 scanning electron microscopy (SEM) with energy-dispersive X-ray (EDX) operating at 15.0 kV to determine the chemical composition of the sample. X-ray diffraction (XRD) analyses were carried out using a Rigaku D/Max-2500/PC X-ray diffractometer with Cu Ka radiation (l ¼ 1.544 A) at a scanning rate of 4 min1 in 2q range from 10 to 80 . Powder X-ray photoelectron spectrometry (XPS) measurements were performed on a PHI5000Versa Probe X-ray photoelectron spectrometer with monochromatic Al Ka radiation.
Results and discussion Microstructure of PteCo@PG catalyst Fig. 1 shows images for GO at different stages. Compared to the pristine GO with a smooth surface (Fig. 1(a)), TEM image of GO-Na2MoO4 calcinated at 650 C (Fig. 1(b)) clearly shows that the carbothermal products are homogeneously attached on graphene sheet. After acid treatment to get rid of metal oxide, PG (Fig. 1(c)) maintains lamellar graphene structure with abundant nano-scaled pores of 10e30 nm across the entire plane. It indicates that the carbothermal products were reacted with GO matrix successfully and existed as insertion for the formation of nanopores. The bared PteCo NPs are irregular spherical shape with clear particle aggregation and the particle diameter is about
3
20 nm (Fig. 2(a)). GO assisted PteCo NPs deposition reveals an improved control to the particle morphology with a slight aggregation (Fig. 2(b)). Furthermore, PteCo particles on PG exhibit an excellent distribution around the matrix nanopores (Fig. 2(c)). It also shows that the PteCo sizes for PteCo@rGO and PteCo@PG are different on uniformity. On the GO surface, the size of PteCo particle is from 5 nm to 15 nm, while that of the NPs on PG is uniformly about 8 nm. This is ascribed to the edge formed by nanopores on PG provide abundant attachment sites which are beneficial to the nucleation of PteCo particles during coprecipitation process. To clarify the formation mechanism of PteCo@PG catalysts, the procedure for fabricating PteCo NPs deposited on nanoporous graphene (PG) sheets is shown in Scheme 1. It contains two important steps, namely, the synthesis of nanoporous graphene sheets, and subsequent deposition of bimetallic PteCo NPs via a facile chemical reduction method. As depicted in the TEM images Fig. 1(aec), the pristine graphene oxide (GO) sheets, present nearly transparent and smooth plane. Nanoporous graphene sheets are fabricated via carbothermal reaction by using Na2MoO4 as etching agent. It is worth noting that in the formation of nanoporous graphene sheets, Na2MoO4 as etching agent after calcination treatment with pristine GO changes into metal oxides adsorbed in the surface. The adjacent carbon atoms of GO is oxidized into CO2. The porous graphene is formed after acid treatment to get rid of metal oxide. The successful deposition of bimetallic PteCo NPs on the PG support is further confirmed by the powder X-ray diffraction (XRD), Raman spectra and X-ray photoelectron spectroscopy, shown in Fig. 3. A broad peak centered at 2q value of 23.62 which characterizes its layer spacing is observed for PG, and the diffraction peak at 10 of the characteristic peak for GO sheet disappears. The changes suggest that the transformation from GO to PG accompanied by the removal of various oxygen-containing functional segments on the GO surface [23]. The XRD peaks of the as-synthesized PteCo@PG catalyst centered at 40.1 , 46.7 and 68.2 are clearly strong, which exhibit the typical Pt face-centered cubic features (PDF # 04-0904), whereas the Co diffraction peaks are invisible. Absence of the Bragg peaks in the XRD pattern is the identification of an amorphous nature of Co in the PteCo bimetallic components [24]. It is worth to mention that the diffraction peak of pure PG at 23.62 moves to 26.34 after loading, indicating that the laying spacing of PG matrix decreased from 3.7662 A to 3.4081 A by the loading of PteCo nanoparticles.
Fig. 1 e TEM images of (a) pristine graphene oxide sheets, (b) GO-Na2MoO4 calcinated at 650 C, (c) nanoporous graphene (PG) sheets. Please cite this article in press as: Ke D, et al., Fabrication of PteCo NPs supported on nanoporous graphene as high-efficient catalyst for hydrolytic dehydrogenation of ammonia borane, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.121
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Fig. 2 e TEM images of (a) PteCo NPs, (b) PteCo@rGO NPs and (c) PteCo@PG NPs.
Scheme 1 e Schematic of PteCo@PG catalysts preparation.
Raman spectra for GO, PG and the composites are shown in Fig. 3 (b). Each spectrum shows two obvious bands centered at 1596 and 1322 cm1, assigned to the G and D bands of the carbon components, respectively. The G band corresponds to the in-plane vibration of sp2 carbon atoms in a 3D hexagonal lattice, while the D band corresponds to the vibrations of sp3 carbon atoms of disordered graphite [25]. On the other hand, the intensity ratio of G and D bands is an important parameter of defect density in the characterization of carbon materials [26]. In the present case, the results indicate that the value of ID/IG changes from 0.77 to 1.16 after the carbothermal reaction for the transformation from GO to PG, which implies the defects increased significantly during the preparation of PG. Also, the substrate defects are further increased after PteCo NPs deposited on the PG scaffold as the value of ID/IG for the PteCo@PG composite is increased to 1.31. It has been verified that the defects on the supporting materials contribute more anchors for metal nanoparticles and have a strong correlation on improving the catalytic activity [19]. Fig. 3(c) shows the XPS survey spectra of GO, PG and PteCo@PG. The high-resolution C1s XPS spectrum (Fig. 3(d)) of as-prepared GO are deconvoluted to five types of carbon bond centered at 284.58, 285.6, 286.7, 287.1, and 288.6 eV are observed, corresponding to sp2C, sp3C, eCeO, eC]O, and e COO groups, respectively [14]. However, after carbothermal reaction, the majority of oxygen functional groups and the sp3 carbon have been removed and the sp2 C bonds with 284.5 eV
are dominant, indicating most of the graphene oxides are reduced and the conjugated graphene networks are restored, which also agrees well with the Raman results. Meanwhile, compared with the bared PG, the C1s XPS spectrum of PteCo@PG centered at 284.7 eV indicates that the structure of nanoporous graphene is unchanged. The positive shift results in the charge interactions between PteCo NPs and PG support (Fig. 3 (e)), which defined as a synergistic effect, have been proved as the principally element to elevate the catalytic performance of the hybrid nanocatalyst to a great extent [27]. Furthermore, XPS studies showed that different Pt and Co states existed in the PteCo@PG catalyst. As illustrated in Fig. 3(e), the most intense doublet of Pt4f spectrum with binding energies of 70.87 and 74.03 eV represents the zerovalent metallic state, developing a chemical shift of 0.22 eV compared to its standard values (71.0 eV). The Co2P3/2 spectra with three binding energy (BE) of 778.6 eV, 781.6 eV and 786.2 eV are observed corresponding to Co metal, Co(II) and Co(III), respectively. The XPS signal of oxidized Co results from oxidation during sample handling for XPS measurements and preservation process [28]. By comparing the BE of metal Co (778.3 eV), we observe a positive shift of 0.3 eV. The increase in binding energy of Co could be attributed to the fact that metal Co acts as an electron donor and contributes partial electrons to Pt, making Co electron-deficient and Pt electron-enriched in their corresponding alloy NPs. The charge transfer from the more electropositive Co elements to Pt implies a downshift of
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Fig. 3 e (a) XRD patterns of GO, PG, PteCo NPs, PteCo@PG NPs, (b) Raman spectra of GO, PG, PteCo@PG NPs, (c) X-ray photoelectron survey spectra (XPS) of GO, PG, PteCo@PG NPs, (d) XPS C1s deconvolution of GO, (e) XPS C1s deconvolution of PG, PteCo@PG NPs, and (f) XPS Pt4f and Co2p deconvolution of PteCo@PG NPs. d-band center of Pt, which is strongly correlated with the adsorption energy, thus affecting the catalytic performance in the catalysis reaction [29].
Catalytic activity of PteCo@PG catalyst on NH3·BH3 hydrolysis Ammonia borane (1.5 mmol, 6 mL) can be catalytically hydrolyzed by freshly prepared nanoporous graphene supported pure Pt NPs, pure Co NPs and PteCo NPs with different composition, respectively. The comparison experiments are carried out under the same conditions at room temperature, which is an ideally mild condition for practical usage. Fig. 4(a) shows the plots of the amount of hydrogen generated during
the hydrolytic dehydrogenation process with the freshly prepared PtxCo1x@PG NPs. Clearly, pure PG shows no catalytic activity for the hydrolysis of AB. PteCo@PG NPs show a much higher H2-release rate than pure Co@PG NPs. The complete hydrolysis of AB finished in less than 3 min for Pt-based catalysts, 9.4 min for Co@PG NPs. Turnover frequency (TOF, molH2 min1 mol1 Pt ) value estimated from the linear portion of the H2 volume/time plot based on total metal and noble metal Pt content are listed in Fig. 4(a), respectively. TOF value is a valid and economic criterion for catalytic activity. Though pure Pt@PG catalyst finished the whole hydrolysis reaction in almost 1 min, suggesting Pt is a more effective element for the hydrolysis of AB, the TOF value based on noble metal Pt content is sharply decreased as compared with PteCo bimetallic
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Fig. 4 e (a) Hydrolysis of aqueous NH3·BH3 solution under ambient atmosphere catalyzed by pure Pt@PG NPs, pure Co@PG NPs, and PteCo@PGNPs with different ratios; (b) Hydrolysis of aqueous NH3·BH3 solution catalyzed by Pt0.1Co0.9 NPs, Pt0.1Co0.9@GO NPs and Pt0.1Co0.9@PG.
catalysts. The catalysis mechanism of PteCo bimetallic NPs could be ascribed to the synergistic effects of Co and Pt caused by charge interactions between PteCo NPs and PG support and the decreased particle size which providing abundance active site [13]. The PteCo synergistic effect could also rely on the atomic ratio of these two elements. It has been reported that according to the volcano shape plotted as a function of the strength of hydrogen-metal bond, a high-efficiency catalyst should have median binding energies (or free energies of adsorption) for reactive intermediates [30,31]. If the bonding energy between metal atoms is too strong, they will restrain the reactivity of reactants and poison the catalyst. Otherwise, if the bonding energy is too weak, the adsorption quantity will be too little, thus lower the catalytic reaction rate [8,32]. Hence, in heterogeneous catalysis for PteCo bimetallic NPs, there is an optimal ratio of Pt and Co to show the highest catalytic activity. The Pt:Co ratio is optimized to achieve the best performance with Pt0.1Co0.9@PG (Pt:Co ¼ 1:9). The optimized Pt0.1Co0.9@PG exhibits markedly high catalytic activity to release a stoichiometric amount of H2 in the hydrolysis of aqueous NH3$BH3 (H2/NH3$BH3 ¼ 3.0) in 3 min with a turnover frequency (TOF) value of 461.17 molH2 min1 mol1 Pt . The excellent performance of Pt0.1Co0.9@PG is evident by comparing its TOF value to those previously reported, as shown in Fig. 5(b).
The supporting effect of nanoporous graphene is clearly demonstrated as compared with bared Pt0.1Co0.9 catalyst and reduced graphene oxide supported Pt0.1Co0.9 catalyst, as shown in Fig. 4 (b). Pt0.1Co0.9 NPs deposited on PG samples show much higher TOF value in sharp contrast to the values of Pt0.1Co0.9@rGO (4 min, 312.46 molH2 min1 mol1 Pt ) and the sample (6 min, 179.66 unsupported Pt0.1Co0.9 molH2 min1 mol1 Pt ). The state-of-the art investigations indicate the enhanced catalytic activity of graphene supported metal NPs should result from the interfacial interaction between metal NPs and graphene materials [33]. The nanoporous graphene sheets provide more edges associated with the existence of holes and contribute more anchors for stabilizing PteCo NPs with homogeneous particle size. These enhancements in the synthesis of nanoporous graphene sheets supported transition metal catalyst will extent the potential applications on hydrogen storage materials, environmental catalysis, electrocatalysis and biotechnology. Fig. 5(a) further illustrates the influence of PG loading percentage on the catalytic performance of Pt0.1Co0.9@PG catalyst for hydrolytic dehydrogenation of NH3$BH3 solution. The effect of PG content on the catalytic activity of the catalysts can be clearly seen in a trend that with increasing PG content, the H2 generation rates gradually increase with more PG loadings up to the optimum value of 30 wt %, then
Fig. 5 e (a) Hydrogen evolution from aqueous NH3·BH3 solution catalyzed by Pt0.1Co0.9@PG-x% (x%: weight percentage of PG support, x ¼ 0e70); (b) A comparison of our Pt0.1Co0.9@PG catalyst with the reported noble metal-based monometallic, bimetallic catalysts in terms of TOF and Ea value in the catalytic dehydrogenation of NH3·BH3. Please cite this article in press as: Ke D, et al., Fabrication of PteCo NPs supported on nanoporous graphene as high-efficient catalyst for hydrolytic dehydrogenation of ammonia borane, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.121
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decrease as the PG loadings further increase. This may be due to that too little PG support is adverse to the dispersion of metal NPs. Agglomeration of excess PteCo NPs on the PG support hinders some of the active site during the hydrolysis reaction. Meanwhile, excessive PG loadings will reduce the content of active metal component and limit the available active sites at the interface of metal and support. As a result, after optimizing the various parameters for the dehydrogenation of NH3$BH3, the best catalyst PteCo@PG with a metal loading of 30 wt% and the atomic ratio of 0.1:0.9 achieved a remarkable TOF value of 461.17 molH2 min1 mol1 Pt . Compared with other ever reported noble metal-based catalysts and Co catalysts (Fig. 5(b)) [18,34e40], the Pt0.1Co0.9@PG catalyst is among the most active catalyst, superior to the majority of noble metal-based catalyst, such as Pd@Co/graphene catalyst [39] and Ru (0)@MWCNT catalyst [34]. We believe that this excellent performance arises from the contribution of the nanoporous graphene sheets which gives PteCo NPs better dispersion, and thus a higher availability of reactive sites, as well as facilitates the charge transfer interaction at the metal-porous graphene interface during the hydrolysis reaction. Fig. 6(a) shows the effect of temperature on the hydrolysis of aqueous NH3$BH3 solution. The values of the rate constant k at different temperatures are calculated from the slope of the linear part of each plot. Under our experimental conditions, k is constant for a given temperature, implying zero order kinetics for the NH3$BH3 hydrolysis reaction. According to the Arrhenius plot of ln(k) versus 1/T (Fig. 6(b)), the activation energy (Ea) value for the hydrolysis is calculated to be 32.79 kJ mol1. Compared with what has been reported on the hydrolytic performance of various Co-, Pt-, and other noble
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metal-based catalysts (Fig. 5(b)), the favorable activation energy value obtained in this work is again an evidence of the auxo-action by Pt0.1Co0.9@PG catalyst to enhance the catalytic activity. We should note that in Fig. 5(b) the completion time for noble metal-based catalysts varies in a wide range due to the nanoparticle size and supporting materials effects; (2) the data obtained from Ag@Co/graphene catalyst [35] is reduced by MeAB and AB and the dehydrogenation of AB is catalyzed by in-situ hydrolysis. As a crucial issue in the practical application of heterogeneous catalyst, the recycle stability of Pt0.1Co0.9@PG catalyst up to fifth run for hydrolysis of AB is tested and shown in Fig. 6(cee). The as-synthesized Pt0.1Co0.9@PG NPs are magnetic and thus bring the advantage of easy separation from the reaction solution by an external magnet. When H2 generation was complete, another equivalent (1.5 mmol) of AB was added to the reaction system and the released gas was once again measured. As shown in Fig. 6(c), there is only a slight decrease in catalytic activity even after five runs of reactions retaining 81.2% of their initial catalytic activity, indicating a wellestablished reusability for practical applications. The slight activity drop is caused by the increase in concentration of metaborate and the viscosity of the solution during the NH3$BH3 hydrolysis [41,42]. Fig. 6(e) shows the TEM image of Pt0.1Co0.9@PG NPs after the fifth run durability test. Clearly, the nanoporous structure and deposited Pt0.1Co0.9 NPs still maintain in good morphologies, and no obvious aggregation and dissolution are observed, which further confirm the good stability of Pt0.1Co0.9 NPs anchored on nanoporous graphene. The results from both hydrolysis activity and stability studies confirm that PteCo@PG is a desirable catalyst for dehydrogenation from NH3$BH3.
Fig. 6 e (a) Plots of time versus H2 generation volume at various temperatures from 293 K to 313 K; (b) Arrhenius plots and TOF values of NH3·BH3 dehydrogenation over 293 Ke313 K; (c) Photographs of the Pt0.1Co0.9@PG dispersions before and after magnetic separation; (d) Hydrolysis of aqueous NH3·BH3 solution catalyzed by Pt0.1Co0.9@PG from the 1st to 5th cycle; (e) TEM image after five cycles. Please cite this article in press as: Ke D, et al., Fabrication of PteCo NPs supported on nanoporous graphene as high-efficient catalyst for hydrolytic dehydrogenation of ammonia borane, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.121
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Conclusions In conclusion, novel PteCo NPs embedded in nanoporous graphene sheets as a high-efficiency catalyst for hydrolytic dehydrogenation of ammonia borane was reported. Nanoporous graphene sheets were fabricated via carbothermal metal oxide etching method, and then PteCo NPs were uniformly embedded in the planes and hole defects. The nanoporous structures not only acted as the anchoring site for stabilizing and dispersing the PteCo NPs, but also yielded a synergistic effect between nanoporous graphene and metal NPs on enhancing the charge transfer and improving catalytic activity and stability. PteCo@PG catalyst exhibited an initial turnover frequency (TOF) of 461.17 molH2 min1 mol1 Pt and retained 81.2% of their initial activity with a complete release of hydrogen in five runs for hydrolytic dehydrogenation of AB, which was superior to most noble metal-based catalysts. Activation energy for the hydrolysis of AB was determined as 32.17 kJ mol1 from the evaluation of kinetic data at different temperatures. Facile preparation and high catalytic performances revealed that PteCo@PG nanohybrids was a promising candidate in developing highly efficient, portable hydrogen generation systems using ammonia borane.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NOs. 21303157 and 51571173), Scientific Research Projects in Colleges and Universities in Hebei Province (QN2016002), and the Innovation Fund for the Graduate Students of Hebei Province (2016SJBS018).
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Please cite this article in press as: Ke D, et al., Fabrication of PteCo NPs supported on nanoporous graphene as high-efficient catalyst for hydrolytic dehydrogenation of ammonia borane, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/ j.ijhydene.2017.09.121