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Monodisperse nickel–palladium alloy nanoparticles supported on reduced graphene oxide as highly efficient catalysts for the hydrolytic dehydrogenation of ammonia borane
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Monodisperse nickelepalladium alloy nanoparticles supported on reduced graphene oxide as highly efficient catalysts for the hydrolytic dehydrogenation of ammonia borane € Nesibe Sedanur C¸iftci, Onder Metin* Department of Chemistry, Faculty of Science, Atatu¨rk University, Erzurum 25240, Turkey
article info
abstract
Article history:
Addressed herein is the catalysis of reduced graphene oxide-supported monodisperse NiPd
Received 4 August 2014
alloy nanoparticles (NPs) (rGO-NiPd) in the hydrolytic dehydrogenation of ammonia borane
Received in revised form
(AB). This is the first example of the use of NiPd alloy NPs as catalyst in the hydrolytic
27 August 2014
dehydrogenation of AB. Monodisperse NiPd alloy NPs (3.5 nm) were synthesized by co-
Accepted 10 September 2014
reduction of nickel(II) acetate and palladium(II) acetylacetonate in oleylamine (OAm) and
Available online 3 October 2014
borane-tert-butylamine complex (BTB) at 100 C. The current recipe allowed to control the composition of NiPd alloy NPs and to study the composition-controlled catalysis of rGO-
Keywords:
NiPd in the hydrolytic dehydrogenation of AB. Among the all compositions tested, the
Nickel
Ni30Pd70 was the most active one with the turnover frequency of 28.7 min1. The rGO-
Palladium
Ni30Pd70 were also durable catalysts in the hydrolytic dehydrogenation of AB providing
Alloy nanoparticles
3650 total turnovers in 35 h and reused at six times without deactivation. The detailed
Reduced graphene oxide
reaction kinetics of hydrolytic dehydrogenation of AB revealed that the reaction proceeds
Ammonia borane
first order with respect to the NiPd concentration and zeroth order with respect to the AB
Dehydrogenation
concentration. The apparent activation energy of the catalytic dehydrogenation of AB was ¼ 45 ± 2 kJ*mol1. also calculated to be Eapp a Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen has been considered to be an alternative energy vector for almost two decades, but it is now in many of our daily life applications owing to the recent advances in nanoscience, catalysis, modeling and bio-inspired approaches that offer new research prospects for a variety of
hydrogen and fuel cell technologies. However, hydrogen storage is still a crucial technology for the advancement of hydrogen and fuel cell power technologies in transportation, stationary and portable applications [1]. Although several hydrogen storage technologies such as compressing gaseous hydrogen are available for mobile systems nowadays, the desired technical requirements have not been accomplished yet. Therefore, the
* Corresponding author. Tel.: þ90 442 231 4410; fax: þ90 442 236 0948. € Metin). E-mail addresses: [email protected], [email protected] (O. http://dx.doi.org/10.1016/j.ijhydene.2014.09.060 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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development of high capacity, efficient and safe hydrogen storage technologies are required [2]. In this respect, several promising storage technologies have been investigated such as chemical hydrogen storage, storing hydrogen within a compound via chemical bond. Among the chemical hydrogen storage materials have been tested so far [3], ammonia borane (H3N.BH3, AB) is the leading candidate owing to its stability in air and solution, very high hydrogen content (19.6 wt%) with a system-level H2 energy storage density of about 2.74 kWh/L (two times higher than DOE's 2017 target, 1.3 kWh/L [2]), and other desirable chemical properties [4]. The hydrogen stored in AB can be generated via several ways (solid state thermolysis [5], dehydrocoupling [6] and dehydrogenation via solvolysis [7]) among which the catalytic hydrolysis (Eq. (1)) appeals to be the most favorable one for mobile applications owing to its favorable technical advantages such as fast hydrogen release and to be performable at low temperatures [8]. Catalyst H3 NBH3 aq þ 2H2 O l ! NH4 BO2 aq þ 3H2 g
(1)
Up to date, many transition metals (Fe [9], Co [10], Ni [11], Cu [12], Pd [13], Ru [14], Pt [15]) or their bimetallic counterparts (NiRu [16], NiFe [17], CuRu [18], CoNi [19], PdPt [20], NiPt [21], CoFe [22], CuFe [23]) have been presented to catalyze the hydrolytic dehydrogenation of AB. Among these catalysts, nickel, cobalt and palladium were emerged to be the most effective ones. In our recent studies, we have reported that monodisperse Ni nanoparticles (NPs) supported on Ketjen carbon [24] and Pd NPs supported on reduced graphene oxide [25] were highly efficient catalysts for the hydrolysis of AB under ambient conditions. Additionally, we have demonstrated that monodisperse CoPd alloy NPs were more efficient catalysts than single Pd NPs owing to the synergistic effect between two distinct metals [26]. Xu and co-workers have also reported the synergistic catalysis of bimetallic Ni-based NPs in the hydrolysis of AB [27]. However, a literature search was resulted in no example of bimetallic NiPd alloy catalysts in the hydrolysis of AB. These results motivated us to study the catalysis of monodiperse NiPd alloy NPs for the hydrolysis of AB. We report herein the catalysis of reduced graphene oxide-supported monodisperse NiPd alloy nanoparticles (NPs) (rGO-NiPd) in the hydrolytic dehydrogenation of AB. To the best of our knowledge, this is the first example of the use of NiPd alloy NPs as catalyst in the hydrolytic dehydrogenation of AB. 3.5 nm NiPd alloy NPs were synthesized by co-reduction of nickel(II) acetate and palladium(II) acetylacetonate in oleylamine (OAm) and borane-tert-butylamine complex (BTB) [28]. The composition of NiPd alloy NPs was easily tuned by changing the metal precursor ratio. Next, as-prepared NiPd alloy NPs were supported on rGO (rGO-NiPd) via simple liquid impregnation method and tested in the hydrolytic dehydrogenation of AB without any special treatment to remove the surfactants. Among the three compositions of NiPd alloy NPs tested, the Ni30Pd70 showed the best performance with the initial turnover frequency (TOF) of 28.7 mol H2 min1. The reaction kinetics of rGO-Ni30Pd70 catalyzed hydrolytic dehydrogenation of AB was studied by depending on the catalyst concentration, substrate concentration and temperature.
Experimental Materials Nickel(II) acetate tetrahydrate (Ni(ac)2.4H2O, 98%), palladium(II) acetylacetonate (Pd(acac)2), oleylamine (OAm, >70%), borane-tert-butylamine (BTB, 97%), 1-octadecene (tech. grade, 90%), hexanes (99%), boraneeammonia complex (AB, 97%), potassium permanganate (KMnO4, >99%), sodium nitrate (NaNO3, >99%), and dimethylformamide (DMF, >99%) were purchased from SigmaeAldrich® and used as received. Hydrogen peroxide (H2O2, 30%) and sulfuric acid (H2SO4, 95e98%) were purchased from Merck®. Natural graphite flakes (average particle size 325 mesh) were purchased from Alfa Aesar®. Deionized water was distilled by water purification system (Milli-Q System). All glassware and Teflon-coated magnetic stir bars were cleaned with acetone, followed by copious rinsing with distilled water before drying at 150 C in oven for overnight.
Characterizations Transmission electron microscope (TEM) images were obtained by using FEI Tecnai G2 Spirit BioTwin High-Contrast microscope instrument operating 120 kV. X-ray diffraction pattern (XRD) was recorded on a Rigaku Miniflex diffractom˚ ), over a 2q range eter with CuKa (30 kV, 15 mA, l ¼ 1.54051 A from 10 to 80 at room temperature. The metal content of the NiPd alloy NPs and rGO-NiPd catalysts was determined by using Leiman series inductively coupled plasma-mass spectroscopy (ICP-MS) after each sample was completely dissolved in aqua-regia (HNO3/HCl: 1/3 v/v ratio).11B NMR spectrum was measured on a Bruker Avance DPX 400 MHz spectrometer (128.2 MHz for 11B NMR).
Synthesis of reduced graphene oxide rGO was prepared by using a well-established two step procedure; (i) the synthesis of graphite oxide via modified Hummer's method [29] and (ii) the reduction of graphene oxide by refluxing its DMF solution for 6 h [30]. The details of the rGO synthesis procedure and the characterization of rGO can be found in our recent reports [31].
Synthesis of monodisperse NiPd alloy nanoparticles and supporting them on reduced graphene oxide The monodisperse NiPd alloy NPs were synthesized by using our established protocol with minor modifications [28]. In a typical synthesis of Ni30Pd70 NPs, nickel(II) acetate tetrahydrate (0.2 mmol) and palladium(II) acetylacetonate (0.2 mmol) were dissolved in 3 mL of OAm in a 20 mL of glass vial. In a four-necked glass reactor that allows to study under inert atmosphere, 200 mg of BTB was dissolved in 3 mL of OAm and 7 mL of 1-octadecene at 100 C under magnetic stirring. Next, the metal precursor mixture was quickly injected into the reactor under argon environment. The reaction was then proceed for 1 h before cooled down to room temperature. Then, the colloidal NPs mixture was transferred into two
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separate centrifuge tube and acetone/ethanol mixture (v/ v ¼ 7/3) was added into the tubes. The NP product was separated by centrifugation at 8500 rpm for 10 min. The NPs were redispersed in hexane and then stored for further use. Ni20Pd80 and Ni40Pd60 NPs were synthesized by using the same protocol but changing the Ni:Pd molar ratio to 0.1:0.25 and 0.3:0.2, respectively. The alloy NPs with the higher Ni content could not be obtained by using the current recipe. In a typical procedure for the preparation of rGO-NiPd catalyst, 75.0 mg of the NiPd NPs (the weight of the NiPd NPs was determined by a simple gravimetric method) were dissolved in 5.0 mL hexane and mixed with 75.0 mg of rGO in ethanol (20 mL). The ethanol/hexane mixture (v/v:7/3) was sonicated for 2 h to ensure complete adsorption of NPs onto rGO. Then, the resultant mixture was centrifuged at 8000 rpm for 10 min and the separated catalyst was washed with ethanol twice and dried under vacuum, giving rGO-NiPd.
Hydrolytic dehydrogenation of ammonia borane catalyzed by rGO-NiPd The activity of rGO-NiPd catalysts were determined by measuring the hydrogen generation rate in a typical waterfilled gas burette system. Before the activity test, 10 mg of rGO-NiPd catalysts were dispersed in 7 mL of distilled water in a round-bottom flask via sonication for 20 min. Next, the aqueous catalyst dispersion was transferred into the jacketed reaction flask (25 mL) containing a teflon-coated magnetic stir bar was placed on a magnetic stirrer (VWR) and thermostated to 25.0 ± 0.5 C. Then, a burette filled with distilled water was connected to the reaction flask via plastic pipe to measure the volume of the hydrogen gas to be generated from the reaction (1 cm water level change equals to 3.4 mL H2 gas under our laboratory conditions). Next, 3 mL of aqueous AB solution (1 mmol AB) was injected into the catalyst solution, the reactor was closed by a septum and the reaction was proceeded at 800 rpm stirring rate. The volume of hydrogen gas generated was measured by recording the displacement of water level at certain time intervals. The ammonia, one of the possible products generated due to the incomplete hydrolysis, was also monitored by using acid/base titration test [24]. To quantify the liberated NH3 gas; the gas generated from the catalytic reaction was passed through 25 mL of standardized 0.001 M HCl solution at room temperature. After gas generation was stopped, the resulted HCl solution was titrated with standard solution of 0.01 M NaOH by using phenolphthalein as indicator. The quantity of the liberated ammonia gas was calculated from the difference between two HCl solutions before and after the reaction.
Kinetics of hydrolytic dehydrogenation of AB catalyzed by rGO-Ni30Pd70 In order to establish the rate law for the hydrolytic dehydrogenation of AB in the presence of rGO-Ni30Pd70 catalyst, three different set of experiments were performed in the same way as described in the Section 2.5. In the first set of experiments, the catalytic reaction was performed at different catalyst concentration (5, 7.5, 10 and 15 mg rGO-Ni30Pd70) by keeping AB concentration at 100 mM and temperature at 25.0 ± 0.5 C.
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The second set of experiments were performed by keeping the catalyst amount constant at 10 mg of rGO-Ni30Pd70 and varying the AB concentration in the range of 50, 100, 200 and 300 mM at 25.0 ± 0.5 C. Finally, the catalytic reaction was performed by using constant AB conc. (100 mM) and catalyst amount (10 mg of rGO-Ni30Pd70) at various temperatures in the range of 293e313 K in order to obtain the activation energy (Ea).
Durability of the rGO-Ni30Pd70 catalysts in the hydrolytic dehydrogenation of AB In a typical reusability test, 20 mg of rGO-NiPd catalysts were isolated from the catalytic reaction solution by filtration after all of the AB present in solution was consumed for each cycle. The isolated rGO-Ni30Pd70 catalysts were washed several times with water to remove the residuals from the surface of the catalyst. Next, a new catalytic reaction was started by dispersing the isolated rGO-Ni30Pd70 catalysts in a solution containing a new batch of 2.0 mmol AB as described in the Section 2.5. A lifetime experiment was also be performed for the determination of the total turnover number (TTON). In a typical lifetime experiment, a catalytic reaction was started as described in the Section 2.5 and a new batch of AB (2 mmol) was added into the reaction mixture after the stoichiometric hydrogen generation was completed without removal of the 10 mg of rGO-Ni30Pd70 catalyst. The lifetime experiments were maintained by addition of new batch of AB at 15 times over time period of 35 h. It has to be noted that the lifetime experiment was stopped after 35 h due to the inconvenience laboratory conditions, but the rGO-Ni30Pd70 catalysts were still be active to generate hydrogen upon addition of a new batch AB.
Results and discussion Monodisperse NiPd NPs were synthesized by using our established protocol which was including a simple organic phase co-reduction of Ni(ac)2 and Pd(acac)2 in the hot mixture of BTB and OAm in ODE solution [28]. A burst co-nucleation of Ni and Pd atoms was prompted and the formation of NiPd alloy NPs with controlled composition was enabled. Fig. 1A and B is the representative TEM images of the as-synthesized Ni30Pd70 NPs from their hexane dispersion taken at different magnifications. As clearly seen from the TEM images and the particle size histogram given as the inset in Fig. 1A, the NiPd NPs have a monodisperse particle size distribution with the mean value of 3.5 ± 0.25 nm. The NPs have a standard deviation of 7.1% in the diameter possessing the monodispersity of NiPd NPs. Because of as-synthesized NiPd NPs were hydrophobic due to the presence OAm as surfactant on the surface of NPs, as-prepared NPs were supported on rGO via simple liquid self-assembly method before their use as catalyst in the hydrolytic dehydrogenation of AB. Fig. 1C shows a typical TEM image for the rGO-Ni30Pd70 catalysts taken from the ethanol dispersion. As seen from Fig. 1C, NiPd NPs were well-dispersed on rGO by preserving their particle size distribution without any observable aggregate formation. The NiPd content of rGO-
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Fig. 1 e TEM images of (A,B) colloidal Ni30Pd70 NPs taken from hexane dispersion at different magnifications, (C) rGONi30Pd70 catalyst, (D) XRD pattern of rGO-Ni30Pd70 catalyst. The inset in Figure 1A shows a particle size histogram of colloidal Ni30Pd70 NPs.
Ni30Pd70 catalysts was determined to be 17.8% wt by ICP-MS analyses performed on different catalyst samples. Fig. 1D shows the XRD pattern of the rGO@Ni30Pd70 catalyst that can be readily indexed to face centered cubic (fcc) phase. The broad peak observed at 2q ¼ 40.1 belongs to (111) plane of fccNiPd and shifted to slightly higher angel compared to single Pd NPs (2q ¼ 39.7 ). These results endorse the alloy formation within NiPd NPs. The peak observed at 2q ¼ 33 belongs to the rGO indicating its few layered structure. Before starting the kinetic studies on rGO-NiPd catalyzed hydrolytic dehydrogenation of AB, the effect of the alloy compositions and support materials on the efficiency of NiPd NPs was investigated. Fig. 2A and B shows the time versus volume of hydrogen generated plots during the hydrolytic dehydrogenation of AB in the presence of rGO-NiPd catalysts with different alloy compositions and supported on different materials (active carbon (AC) and nano-alumina (Al2O3), Figure S1), respectively. As depicted by Fig. 2A, the rGONi30Pd70 provided the best initial TOF value of 28.7 min1 among the three alloy compositions tested in the reaction at room temperature (See Table S1 to compare the activity of rGO-Ni30Pd70 catalyst with very recently reported bimetallic alloy NPs comprising at least one first-row transition metal). On the other hand, the Ni30Pd70 NPs supported on either active carbon (AC-Ni30Pd70) (Figure S1A) or aluminum oxide
nanopowder (Al2O3eNi30Pd70) (Figure S1B) were active catalysts for the hydrolytic dehydrogenation of AB, but provided lower initial TOF values (9.6 min1 for AC-Ni30Pd70 and 6.3 min1 for Al2O3eNi30Pd70) compared to one obtained by rGO-Ni30Pd70. Upon these results, the rGO-Ni30Pd70 was selected the best catalyst in terms of the initial TOF value for the hydrolysis of AB and used for the kinetic studies. Additionally, the TOF of rGO-Ni30Pd70 catalysts is higher than the one provided by single Pd NPs (TOF (rGO-Pd) ¼ 26.6 min1 [24]) in the hydrolytic dehydrogenation of AB although it includes 30% less Pd atoms, which means rGO@Ni30Pd70 is considered to be a noble-metal economical catalyst. To evaluate the role of rGO support on the high activity of NiPd alloy NPs, we performed a test reaction on the AB hydrolysis in the presence of only rGO. However, no gas generation was observed by rGO catalyzed AB hydrolysis over 2 h which reveals that NiPd alloy NPs are the active catalysts in hydrogen generation from AB hydrolysis. The reaction kinetics of rGO-Ni30Pd70 catalyzed hydrolytic dehydrogenation of AB was studied at different catalysts and AB concentrations, and at various temperatures. Fig. 3A shows the time versus volume of hydrogen generated plots during the hydrolysis of AB (100 mM) at different rGO-Ni30Pd70 concentrations (5e15 mg, 0.5e1.5 mM NiPd) at 25 ± 0.5 C. The initial hydrogen generation rate was determined from the
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Fig. 2 e Time versus volume of hydrogen generated by the rGO-NiPd catalyzed AB hydrolysis at various NiPd alloy compositions (A) and in the presence of different support materials (B).
Fig. 3 e Time versus volume of hydrogen generated by the rGO-Ni30Pd70 catalyzed AB hydrolysis at different catalyst concentrations (A) and at different AB concentrations (B). The insets show the ln rate versus ln [NiPd] or ln [AB] plots.
Fig. 4 e Time versus volume of hydrogen generated by the rGO-Ni30Pd70 catalyzed AB hydrolysis at various temperatures (T ¼ 293e313 K). The inset shows the Arrhenius plot (ln k vs (1/T)) for the calculation of apparent activation energy.
linear portion of the plot for each catalyst concentration. As it would be expected, the hydrogen generation rate shows a linear increase by increasing catalyst concentration which can be depicted by the line with a slope of 1.01 in the inset of Fig. 3A that shows the plot of catalyst concentration vs hydrogen generation rate, both in logarithmic scale. Upon these results, the first order kinetics with respect to the catalyst concentration for the hydrolysis of AB in the presence of rGO-Ni30Pd70 catalyst can be concluded. On the other hand, the H2 generation rate was found to be practically independent of AB concentration (Fig. 3B). The line with a slope of 0.02 obtained by a plot of H2 generation rate versus AB concentration, both in logarithmic scale (the inset in Fig. 3B), indicates that the rGO-Ni30Pd70 catalyzed AB hydrolysis is zeroth-order with respect to AB concentration. This result is not surprising considering our experimental conditions in which at least an AB/catalyst ratio of 100 was used. As a conclusion, the rate law of rGO-Ni30Pd70 catalyzed hydrolytic dehydrogenation of AB can be written as rate ¼ kapp[NiPd]. In the final set of kinetic studies, the rGO-Ni30Pd70 catalyzed hydrolysis of AB was performed at various temperatures
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Fig. 5 e Time versus volume of hydrogen generated by the rGO-Ni30Pd70 (1 mM NiPd) catalyzed AB (100 mM) hydrolysis during the six runs reusability test (A) and the catalytic lifetime experiment over 35 h (B). The inset in Figure 5A shows a representative TEM image for the rGO-Ni30Pd70 catalyst after the six runs reusability test.
(293e313 K) by keeping rGO-Ni30Pd70 concentration constant at 1.0 mM NiPd and AB concentration at 100 mM. The apparent rate constants kapp at different temperatures were calculated from the slope of initial linear region of each plot in Fig. 4 by using rate law equation and used to draw Arrhenius plot (the inset in Fig. 4). The apparent activation energy was calculated ¼ 45 ± 2 kJ*mol1 from the Arrhenius plot, which is to be Eapp a comparable to the ones reported in the literature. An extra benefit of using the new rGO-Ni30Pd70 catalyst in the hydrolytic dehydrogenation of AB is that it is stable and reusable. We tested the durability of rGO-Ni30Pd70 catalysts in the hydrolysis of AB performing the reusability and recyclability tests. Fig. 5A shows the time vs volume of hydrogen generated plots during the six runs reusability test starting with 20 mg of rGO-Ni30Pd70 catalysts (1 mM NiPd) and AB (100 mM) at room temperature. A very sharp decrease in the initial TOF of rGO-Ni30Pd70 catalysts from 28.7 min1 to 18.2 min1 was observed by 3rd run, but then it kept almost constant at 18 min1. The inset of Fig. 5A shows a representative TEM image of rGO-Ni30Pd70 catalysts after the six runs reusability test. As clearly seen from the TEM image, there is no noticeable change in the morphology and particle size of Ni30Pd70 NPs on rGO after catalysis. Depending on these results, we believe that the sharp drop in the activity of rGO-Ni30Pd70 catalyst in first three runs is due to the loose of some catalysts during the centrifugation or another part of separation process due to the high surface area and very thin-layered structure of rGO, which will be tolerated if a better separation system is integrated. Our studies on the magnetically separable rGONPs catalysts are under investigation in our research laboratory. The NH3 gas generation is a common problem for the catalytic AB hydrolysis. In this regard, to quantify whether NH3 gas is generated during the rGO-NiPd catalyzed AB hydrolysis, we performed acid/base titration test [24]. The results revealed that no detectable amount of NH3 gas was generated during the catalytic AB hydrolysis indicating the complete hydrolysis of AB complex into the hydrogen gas.
Conclusions In summary, we have shown for the first time that rGO-NiPd is a highly active catalyst for hydrolytic dehydrogenation of AB as well as being noble-metal economical and reusable. The NiPd NPs show a composition-dependent catalysis with Ni30Pd70 NPs being the most active at room temperature. The kinetic studies on rGO-Ni30Pd70 catalyzed AB dehydrogenation revealed that it is first order with respect to the catalyst concentration and zeroth order with respect to AB concentration. The apparent activation energy of the rGO-Ni30Pd70 catalyzed AB dehydrogenation is found to be 45 kJ mol1. Furthermore, the rGO-Ni30Pd70 catalyst has also shown an excellent catalytic cycle lifetime by providing a TTON number of 3600 over 35 h and will provide higher at longer reaction times. The rGO-Ni30Pd70 is a promising catalyst in replacing pure noble metals (Pd, Ru, Rh, Pt) for the practical AB hydrolysis as a chemical hydrogen storage tool and to be employed for various reactions.
Acknowledgments This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK, Project No: 113Z276) and Atatu¨rk University Scientific Research Project Council (Project No: 2013/88).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.09.060.
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2009;34:3816e22; € Ozkar € (b)Metin O, S. Water soluble nickel(0) and cobalt(0) nanoclusters stabilized by poly(4-styrenesulfonic acid-comaleic acid): highly active, durable and cost-effective catalysts in hydrogen generation from the hydrolysis of ammonia borane. Int J Hydrogen Energy 2011;36:1424e32. Yang Y, Lu Z-H, Hu Y, Zhang Z, Hi W, Chen X, et al. Facile in situ synthesis of copper nanoparticles supported on reduced graphene oxide for hydrolytic dehydrogenation of ammonia borane. RSC Adv 2014;4:13749e52. Akbayrak S, Kaya M, Volkan M, Ozkar S. Palladium(0) nanoparticles supported on silica-coated cobalt ferrite: a highly active, magnetically isolable and reusable catalyst for hydrolytic dehydrogenation of ammonia borane. Appl Catal B Env 2014;147:387e93. € Facile synthesis of nearly monodisperse Can H, Metin O. ruthenium nanoparticles and their catalysis in the hydrolytic dehydrogenation of ammonia borane for chemical hydrogen storage. Appl Catal B Env 2012;125:304e10. Aijaz A, Karkamkar A, Choi YJ, Tsumori N, Ronnebro E, Autrey T, et al. Immobilizing highly catalytically active Pt nanoparticles inside the pores of metaleorganic framework: a double solvents approach. J Am Chem Soc 2012;134:13926e9. Chen GZ, Desinan S, Rosei R, Rosei F, Ma DL. Synthesis of NieRu alloy nanoparticles and their high catalytic activity in dehydrogenation of ammonia borane. Chem Eur J 2012;18:7925e30. Lai SW, Lin HL, Lin YP, Yu TL. Hydrolysis of ammoniaeborane catalyzed by an ironenickel alloy on an SBA-15 support. Int J Hydrogen Energy 2013;38:4636e47. Cao N, Hu K, Luo W, Cheng GZ. RuCu nanoparticles supported on graphene: a highly efficient catalyst for hydrolysis of ammonia borane. J Alloy Comp 2014;590:241e6. Feng WQ, Yang L, Cao N, Du C, Dai HM, Luo W, et al. In situ facile synthesis of bimetallic CoNi catalyst supported on graphene for hydrolytic dehydrogenation of amine borane. Int J Hydrogenen Energy 2014;39:3371e80. Amali AJ, Aranishi K, Uchida T, Xu Q. PdPt nanocubes: a high-performance catalyst for hydrolytic dehydrogenation of ammonia borane. Part Part Syst Char 2013;30:888e92. Aranishi K, Singh AK, Xu Q. Dendrimer-encapsulated bimetallic Pt-Ni nanoparticles as highly efficient catalysts for hydrogen generation from chemical hydrogen storage materials. ChemCatChem 2013;5:2248e52. Qiu FY, Li L, Liu G, Wang YJ, Wang YP, An CH, et al. In situ synthesized FeeCo/C nano-alloys as catalysts for the hydrolysis of ammonia borane. Int J Hydrogen Energy 2013;38:3241e9. Lu ZH, Yao QL, Huang W, Zhou RY, Zhou RF, Chen XS. Catalytic hydrolysis of ammonia borane via magnetically recyclable copper iron nanoparticles for chemical hydrogen storage. Int J Hydrogen Energy 2013;38:5330e7. € Mazumder V, Ozkar € Metin O, S, Sun S. Monodisperse nickel nanoparticles and their catalysis in hydrolytic dehydrogenation of ammonia borane. J Am Chem Soc 2010;32:1468e9. € Hydrolytic dehydrogenation of Kılıc¸ B, Sencanlı S, Metin O. ammonia borane catalyzed by reduced graphene oxide supported monodisperse palladium nanoparticles: high activity and detailed reaction kinetics. J Mol Cat A Chem 2012;361e362:104e10. € Sun S. Catalytic hydrolysis of Sun D, Mazumder V, Metin O, ammonia borane via cobalt palladium nanoparticles. ACS Nano 2011;5:6458e64. Jiang H-L, Umegaki T, Akita T, Zhang X-B, Haruta M, Xu Q. Bimetallic AueNi nanoparticles embedded in sio2 nanospheres: synergetic catalysis in hydrolytic
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Monodisperse nickelepalladium alloy nanoparticles supported on reduced graphene oxide as highly efficient catalysts for the hydrolytic dehydrogenation of ammonia borane € Nesibe Sedanur C¸iftci, Onder Metin* Department of Chemistry, Faculty of Science, Atatu¨rk University, Erzurum 25240, Turkey
article info
abstract
Article history:
Addressed herein is the catalysis of reduced graphene oxide-supported monodisperse NiPd
Received 4 August 2014
alloy nanoparticles (NPs) (rGO-NiPd) in the hydrolytic dehydrogenation of ammonia borane
Received in revised form
(AB). This is the first example of the use of NiPd alloy NPs as catalyst in the hydrolytic
27 August 2014
dehydrogenation of AB. Monodisperse NiPd alloy NPs (3.5 nm) were synthesized by co-
Accepted 10 September 2014
reduction of nickel(II) acetate and palladium(II) acetylacetonate in oleylamine (OAm) and
Available online 3 October 2014
borane-tert-butylamine complex (BTB) at 100 C. The current recipe allowed to control the composition of NiPd alloy NPs and to study the composition-controlled catalysis of rGO-
Keywords:
NiPd in the hydrolytic dehydrogenation of AB. Among the all compositions tested, the
Nickel
Ni30Pd70 was the most active one with the turnover frequency of 28.7 min1. The rGO-
Palladium
Ni30Pd70 were also durable catalysts in the hydrolytic dehydrogenation of AB providing
Alloy nanoparticles
3650 total turnovers in 35 h and reused at six times without deactivation. The detailed
Reduced graphene oxide
reaction kinetics of hydrolytic dehydrogenation of AB revealed that the reaction proceeds
Ammonia borane
first order with respect to the NiPd concentration and zeroth order with respect to the AB
Dehydrogenation
concentration. The apparent activation energy of the catalytic dehydrogenation of AB was ¼ 45 ± 2 kJ*mol1. also calculated to be Eapp a Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen has been considered to be an alternative energy vector for almost two decades, but it is now in many of our daily life applications owing to the recent advances in nanoscience, catalysis, modeling and bio-inspired approaches that offer new research prospects for a variety of
hydrogen and fuel cell technologies. However, hydrogen storage is still a crucial technology for the advancement of hydrogen and fuel cell power technologies in transportation, stationary and portable applications [1]. Although several hydrogen storage technologies such as compressing gaseous hydrogen are available for mobile systems nowadays, the desired technical requirements have not been accomplished yet. Therefore, the
* Corresponding author. Tel.: þ90 442 231 4410; fax: þ90 442 236 0948. € Metin). E-mail addresses: [email protected], [email protected] (O. http://dx.doi.org/10.1016/j.ijhydene.2014.09.060 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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development of high capacity, efficient and safe hydrogen storage technologies are required [2]. In this respect, several promising storage technologies have been investigated such as chemical hydrogen storage, storing hydrogen within a compound via chemical bond. Among the chemical hydrogen storage materials have been tested so far [3], ammonia borane (H3N.BH3, AB) is the leading candidate owing to its stability in air and solution, very high hydrogen content (19.6 wt%) with a system-level H2 energy storage density of about 2.74 kWh/L (two times higher than DOE's 2017 target, 1.3 kWh/L [2]), and other desirable chemical properties [4]. The hydrogen stored in AB can be generated via several ways (solid state thermolysis [5], dehydrocoupling [6] and dehydrogenation via solvolysis [7]) among which the catalytic hydrolysis (Eq. (1)) appeals to be the most favorable one for mobile applications owing to its favorable technical advantages such as fast hydrogen release and to be performable at low temperatures [8]. Catalyst H3 NBH3 aq þ 2H2 O l ! NH4 BO2 aq þ 3H2 g
(1)
Up to date, many transition metals (Fe [9], Co [10], Ni [11], Cu [12], Pd [13], Ru [14], Pt [15]) or their bimetallic counterparts (NiRu [16], NiFe [17], CuRu [18], CoNi [19], PdPt [20], NiPt [21], CoFe [22], CuFe [23]) have been presented to catalyze the hydrolytic dehydrogenation of AB. Among these catalysts, nickel, cobalt and palladium were emerged to be the most effective ones. In our recent studies, we have reported that monodisperse Ni nanoparticles (NPs) supported on Ketjen carbon [24] and Pd NPs supported on reduced graphene oxide [25] were highly efficient catalysts for the hydrolysis of AB under ambient conditions. Additionally, we have demonstrated that monodisperse CoPd alloy NPs were more efficient catalysts than single Pd NPs owing to the synergistic effect between two distinct metals [26]. Xu and co-workers have also reported the synergistic catalysis of bimetallic Ni-based NPs in the hydrolysis of AB [27]. However, a literature search was resulted in no example of bimetallic NiPd alloy catalysts in the hydrolysis of AB. These results motivated us to study the catalysis of monodiperse NiPd alloy NPs for the hydrolysis of AB. We report herein the catalysis of reduced graphene oxide-supported monodisperse NiPd alloy nanoparticles (NPs) (rGO-NiPd) in the hydrolytic dehydrogenation of AB. To the best of our knowledge, this is the first example of the use of NiPd alloy NPs as catalyst in the hydrolytic dehydrogenation of AB. 3.5 nm NiPd alloy NPs were synthesized by co-reduction of nickel(II) acetate and palladium(II) acetylacetonate in oleylamine (OAm) and borane-tert-butylamine complex (BTB) [28]. The composition of NiPd alloy NPs was easily tuned by changing the metal precursor ratio. Next, as-prepared NiPd alloy NPs were supported on rGO (rGO-NiPd) via simple liquid impregnation method and tested in the hydrolytic dehydrogenation of AB without any special treatment to remove the surfactants. Among the three compositions of NiPd alloy NPs tested, the Ni30Pd70 showed the best performance with the initial turnover frequency (TOF) of 28.7 mol H2 min1. The reaction kinetics of rGO-Ni30Pd70 catalyzed hydrolytic dehydrogenation of AB was studied by depending on the catalyst concentration, substrate concentration and temperature.
Experimental Materials Nickel(II) acetate tetrahydrate (Ni(ac)2.4H2O, 98%), palladium(II) acetylacetonate (Pd(acac)2), oleylamine (OAm, >70%), borane-tert-butylamine (BTB, 97%), 1-octadecene (tech. grade, 90%), hexanes (99%), boraneeammonia complex (AB, 97%), potassium permanganate (KMnO4, >99%), sodium nitrate (NaNO3, >99%), and dimethylformamide (DMF, >99%) were purchased from SigmaeAldrich® and used as received. Hydrogen peroxide (H2O2, 30%) and sulfuric acid (H2SO4, 95e98%) were purchased from Merck®. Natural graphite flakes (average particle size 325 mesh) were purchased from Alfa Aesar®. Deionized water was distilled by water purification system (Milli-Q System). All glassware and Teflon-coated magnetic stir bars were cleaned with acetone, followed by copious rinsing with distilled water before drying at 150 C in oven for overnight.
Characterizations Transmission electron microscope (TEM) images were obtained by using FEI Tecnai G2 Spirit BioTwin High-Contrast microscope instrument operating 120 kV. X-ray diffraction pattern (XRD) was recorded on a Rigaku Miniflex diffractom˚ ), over a 2q range eter with CuKa (30 kV, 15 mA, l ¼ 1.54051 A from 10 to 80 at room temperature. The metal content of the NiPd alloy NPs and rGO-NiPd catalysts was determined by using Leiman series inductively coupled plasma-mass spectroscopy (ICP-MS) after each sample was completely dissolved in aqua-regia (HNO3/HCl: 1/3 v/v ratio).11B NMR spectrum was measured on a Bruker Avance DPX 400 MHz spectrometer (128.2 MHz for 11B NMR).
Synthesis of reduced graphene oxide rGO was prepared by using a well-established two step procedure; (i) the synthesis of graphite oxide via modified Hummer's method [29] and (ii) the reduction of graphene oxide by refluxing its DMF solution for 6 h [30]. The details of the rGO synthesis procedure and the characterization of rGO can be found in our recent reports [31].
Synthesis of monodisperse NiPd alloy nanoparticles and supporting them on reduced graphene oxide The monodisperse NiPd alloy NPs were synthesized by using our established protocol with minor modifications [28]. In a typical synthesis of Ni30Pd70 NPs, nickel(II) acetate tetrahydrate (0.2 mmol) and palladium(II) acetylacetonate (0.2 mmol) were dissolved in 3 mL of OAm in a 20 mL of glass vial. In a four-necked glass reactor that allows to study under inert atmosphere, 200 mg of BTB was dissolved in 3 mL of OAm and 7 mL of 1-octadecene at 100 C under magnetic stirring. Next, the metal precursor mixture was quickly injected into the reactor under argon environment. The reaction was then proceed for 1 h before cooled down to room temperature. Then, the colloidal NPs mixture was transferred into two
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separate centrifuge tube and acetone/ethanol mixture (v/ v ¼ 7/3) was added into the tubes. The NP product was separated by centrifugation at 8500 rpm for 10 min. The NPs were redispersed in hexane and then stored for further use. Ni20Pd80 and Ni40Pd60 NPs were synthesized by using the same protocol but changing the Ni:Pd molar ratio to 0.1:0.25 and 0.3:0.2, respectively. The alloy NPs with the higher Ni content could not be obtained by using the current recipe. In a typical procedure for the preparation of rGO-NiPd catalyst, 75.0 mg of the NiPd NPs (the weight of the NiPd NPs was determined by a simple gravimetric method) were dissolved in 5.0 mL hexane and mixed with 75.0 mg of rGO in ethanol (20 mL). The ethanol/hexane mixture (v/v:7/3) was sonicated for 2 h to ensure complete adsorption of NPs onto rGO. Then, the resultant mixture was centrifuged at 8000 rpm for 10 min and the separated catalyst was washed with ethanol twice and dried under vacuum, giving rGO-NiPd.
Hydrolytic dehydrogenation of ammonia borane catalyzed by rGO-NiPd The activity of rGO-NiPd catalysts were determined by measuring the hydrogen generation rate in a typical waterfilled gas burette system. Before the activity test, 10 mg of rGO-NiPd catalysts were dispersed in 7 mL of distilled water in a round-bottom flask via sonication for 20 min. Next, the aqueous catalyst dispersion was transferred into the jacketed reaction flask (25 mL) containing a teflon-coated magnetic stir bar was placed on a magnetic stirrer (VWR) and thermostated to 25.0 ± 0.5 C. Then, a burette filled with distilled water was connected to the reaction flask via plastic pipe to measure the volume of the hydrogen gas to be generated from the reaction (1 cm water level change equals to 3.4 mL H2 gas under our laboratory conditions). Next, 3 mL of aqueous AB solution (1 mmol AB) was injected into the catalyst solution, the reactor was closed by a septum and the reaction was proceeded at 800 rpm stirring rate. The volume of hydrogen gas generated was measured by recording the displacement of water level at certain time intervals. The ammonia, one of the possible products generated due to the incomplete hydrolysis, was also monitored by using acid/base titration test [24]. To quantify the liberated NH3 gas; the gas generated from the catalytic reaction was passed through 25 mL of standardized 0.001 M HCl solution at room temperature. After gas generation was stopped, the resulted HCl solution was titrated with standard solution of 0.01 M NaOH by using phenolphthalein as indicator. The quantity of the liberated ammonia gas was calculated from the difference between two HCl solutions before and after the reaction.
Kinetics of hydrolytic dehydrogenation of AB catalyzed by rGO-Ni30Pd70 In order to establish the rate law for the hydrolytic dehydrogenation of AB in the presence of rGO-Ni30Pd70 catalyst, three different set of experiments were performed in the same way as described in the Section 2.5. In the first set of experiments, the catalytic reaction was performed at different catalyst concentration (5, 7.5, 10 and 15 mg rGO-Ni30Pd70) by keeping AB concentration at 100 mM and temperature at 25.0 ± 0.5 C.
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The second set of experiments were performed by keeping the catalyst amount constant at 10 mg of rGO-Ni30Pd70 and varying the AB concentration in the range of 50, 100, 200 and 300 mM at 25.0 ± 0.5 C. Finally, the catalytic reaction was performed by using constant AB conc. (100 mM) and catalyst amount (10 mg of rGO-Ni30Pd70) at various temperatures in the range of 293e313 K in order to obtain the activation energy (Ea).
Durability of the rGO-Ni30Pd70 catalysts in the hydrolytic dehydrogenation of AB In a typical reusability test, 20 mg of rGO-NiPd catalysts were isolated from the catalytic reaction solution by filtration after all of the AB present in solution was consumed for each cycle. The isolated rGO-Ni30Pd70 catalysts were washed several times with water to remove the residuals from the surface of the catalyst. Next, a new catalytic reaction was started by dispersing the isolated rGO-Ni30Pd70 catalysts in a solution containing a new batch of 2.0 mmol AB as described in the Section 2.5. A lifetime experiment was also be performed for the determination of the total turnover number (TTON). In a typical lifetime experiment, a catalytic reaction was started as described in the Section 2.5 and a new batch of AB (2 mmol) was added into the reaction mixture after the stoichiometric hydrogen generation was completed without removal of the 10 mg of rGO-Ni30Pd70 catalyst. The lifetime experiments were maintained by addition of new batch of AB at 15 times over time period of 35 h. It has to be noted that the lifetime experiment was stopped after 35 h due to the inconvenience laboratory conditions, but the rGO-Ni30Pd70 catalysts were still be active to generate hydrogen upon addition of a new batch AB.
Results and discussion Monodisperse NiPd NPs were synthesized by using our established protocol which was including a simple organic phase co-reduction of Ni(ac)2 and Pd(acac)2 in the hot mixture of BTB and OAm in ODE solution [28]. A burst co-nucleation of Ni and Pd atoms was prompted and the formation of NiPd alloy NPs with controlled composition was enabled. Fig. 1A and B is the representative TEM images of the as-synthesized Ni30Pd70 NPs from their hexane dispersion taken at different magnifications. As clearly seen from the TEM images and the particle size histogram given as the inset in Fig. 1A, the NiPd NPs have a monodisperse particle size distribution with the mean value of 3.5 ± 0.25 nm. The NPs have a standard deviation of 7.1% in the diameter possessing the monodispersity of NiPd NPs. Because of as-synthesized NiPd NPs were hydrophobic due to the presence OAm as surfactant on the surface of NPs, as-prepared NPs were supported on rGO via simple liquid self-assembly method before their use as catalyst in the hydrolytic dehydrogenation of AB. Fig. 1C shows a typical TEM image for the rGO-Ni30Pd70 catalysts taken from the ethanol dispersion. As seen from Fig. 1C, NiPd NPs were well-dispersed on rGO by preserving their particle size distribution without any observable aggregate formation. The NiPd content of rGO-
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Fig. 1 e TEM images of (A,B) colloidal Ni30Pd70 NPs taken from hexane dispersion at different magnifications, (C) rGONi30Pd70 catalyst, (D) XRD pattern of rGO-Ni30Pd70 catalyst. The inset in Figure 1A shows a particle size histogram of colloidal Ni30Pd70 NPs.
Ni30Pd70 catalysts was determined to be 17.8% wt by ICP-MS analyses performed on different catalyst samples. Fig. 1D shows the XRD pattern of the rGO@Ni30Pd70 catalyst that can be readily indexed to face centered cubic (fcc) phase. The broad peak observed at 2q ¼ 40.1 belongs to (111) plane of fccNiPd and shifted to slightly higher angel compared to single Pd NPs (2q ¼ 39.7 ). These results endorse the alloy formation within NiPd NPs. The peak observed at 2q ¼ 33 belongs to the rGO indicating its few layered structure. Before starting the kinetic studies on rGO-NiPd catalyzed hydrolytic dehydrogenation of AB, the effect of the alloy compositions and support materials on the efficiency of NiPd NPs was investigated. Fig. 2A and B shows the time versus volume of hydrogen generated plots during the hydrolytic dehydrogenation of AB in the presence of rGO-NiPd catalysts with different alloy compositions and supported on different materials (active carbon (AC) and nano-alumina (Al2O3), Figure S1), respectively. As depicted by Fig. 2A, the rGONi30Pd70 provided the best initial TOF value of 28.7 min1 among the three alloy compositions tested in the reaction at room temperature (See Table S1 to compare the activity of rGO-Ni30Pd70 catalyst with very recently reported bimetallic alloy NPs comprising at least one first-row transition metal). On the other hand, the Ni30Pd70 NPs supported on either active carbon (AC-Ni30Pd70) (Figure S1A) or aluminum oxide
nanopowder (Al2O3eNi30Pd70) (Figure S1B) were active catalysts for the hydrolytic dehydrogenation of AB, but provided lower initial TOF values (9.6 min1 for AC-Ni30Pd70 and 6.3 min1 for Al2O3eNi30Pd70) compared to one obtained by rGO-Ni30Pd70. Upon these results, the rGO-Ni30Pd70 was selected the best catalyst in terms of the initial TOF value for the hydrolysis of AB and used for the kinetic studies. Additionally, the TOF of rGO-Ni30Pd70 catalysts is higher than the one provided by single Pd NPs (TOF (rGO-Pd) ¼ 26.6 min1 [24]) in the hydrolytic dehydrogenation of AB although it includes 30% less Pd atoms, which means rGO@Ni30Pd70 is considered to be a noble-metal economical catalyst. To evaluate the role of rGO support on the high activity of NiPd alloy NPs, we performed a test reaction on the AB hydrolysis in the presence of only rGO. However, no gas generation was observed by rGO catalyzed AB hydrolysis over 2 h which reveals that NiPd alloy NPs are the active catalysts in hydrogen generation from AB hydrolysis. The reaction kinetics of rGO-Ni30Pd70 catalyzed hydrolytic dehydrogenation of AB was studied at different catalysts and AB concentrations, and at various temperatures. Fig. 3A shows the time versus volume of hydrogen generated plots during the hydrolysis of AB (100 mM) at different rGO-Ni30Pd70 concentrations (5e15 mg, 0.5e1.5 mM NiPd) at 25 ± 0.5 C. The initial hydrogen generation rate was determined from the
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Fig. 2 e Time versus volume of hydrogen generated by the rGO-NiPd catalyzed AB hydrolysis at various NiPd alloy compositions (A) and in the presence of different support materials (B).
Fig. 3 e Time versus volume of hydrogen generated by the rGO-Ni30Pd70 catalyzed AB hydrolysis at different catalyst concentrations (A) and at different AB concentrations (B). The insets show the ln rate versus ln [NiPd] or ln [AB] plots.
Fig. 4 e Time versus volume of hydrogen generated by the rGO-Ni30Pd70 catalyzed AB hydrolysis at various temperatures (T ¼ 293e313 K). The inset shows the Arrhenius plot (ln k vs (1/T)) for the calculation of apparent activation energy.
linear portion of the plot for each catalyst concentration. As it would be expected, the hydrogen generation rate shows a linear increase by increasing catalyst concentration which can be depicted by the line with a slope of 1.01 in the inset of Fig. 3A that shows the plot of catalyst concentration vs hydrogen generation rate, both in logarithmic scale. Upon these results, the first order kinetics with respect to the catalyst concentration for the hydrolysis of AB in the presence of rGO-Ni30Pd70 catalyst can be concluded. On the other hand, the H2 generation rate was found to be practically independent of AB concentration (Fig. 3B). The line with a slope of 0.02 obtained by a plot of H2 generation rate versus AB concentration, both in logarithmic scale (the inset in Fig. 3B), indicates that the rGO-Ni30Pd70 catalyzed AB hydrolysis is zeroth-order with respect to AB concentration. This result is not surprising considering our experimental conditions in which at least an AB/catalyst ratio of 100 was used. As a conclusion, the rate law of rGO-Ni30Pd70 catalyzed hydrolytic dehydrogenation of AB can be written as rate ¼ kapp[NiPd]. In the final set of kinetic studies, the rGO-Ni30Pd70 catalyzed hydrolysis of AB was performed at various temperatures
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Fig. 5 e Time versus volume of hydrogen generated by the rGO-Ni30Pd70 (1 mM NiPd) catalyzed AB (100 mM) hydrolysis during the six runs reusability test (A) and the catalytic lifetime experiment over 35 h (B). The inset in Figure 5A shows a representative TEM image for the rGO-Ni30Pd70 catalyst after the six runs reusability test.
(293e313 K) by keeping rGO-Ni30Pd70 concentration constant at 1.0 mM NiPd and AB concentration at 100 mM. The apparent rate constants kapp at different temperatures were calculated from the slope of initial linear region of each plot in Fig. 4 by using rate law equation and used to draw Arrhenius plot (the inset in Fig. 4). The apparent activation energy was calculated ¼ 45 ± 2 kJ*mol1 from the Arrhenius plot, which is to be Eapp a comparable to the ones reported in the literature. An extra benefit of using the new rGO-Ni30Pd70 catalyst in the hydrolytic dehydrogenation of AB is that it is stable and reusable. We tested the durability of rGO-Ni30Pd70 catalysts in the hydrolysis of AB performing the reusability and recyclability tests. Fig. 5A shows the time vs volume of hydrogen generated plots during the six runs reusability test starting with 20 mg of rGO-Ni30Pd70 catalysts (1 mM NiPd) and AB (100 mM) at room temperature. A very sharp decrease in the initial TOF of rGO-Ni30Pd70 catalysts from 28.7 min1 to 18.2 min1 was observed by 3rd run, but then it kept almost constant at 18 min1. The inset of Fig. 5A shows a representative TEM image of rGO-Ni30Pd70 catalysts after the six runs reusability test. As clearly seen from the TEM image, there is no noticeable change in the morphology and particle size of Ni30Pd70 NPs on rGO after catalysis. Depending on these results, we believe that the sharp drop in the activity of rGO-Ni30Pd70 catalyst in first three runs is due to the loose of some catalysts during the centrifugation or another part of separation process due to the high surface area and very thin-layered structure of rGO, which will be tolerated if a better separation system is integrated. Our studies on the magnetically separable rGONPs catalysts are under investigation in our research laboratory. The NH3 gas generation is a common problem for the catalytic AB hydrolysis. In this regard, to quantify whether NH3 gas is generated during the rGO-NiPd catalyzed AB hydrolysis, we performed acid/base titration test [24]. The results revealed that no detectable amount of NH3 gas was generated during the catalytic AB hydrolysis indicating the complete hydrolysis of AB complex into the hydrogen gas.
Conclusions In summary, we have shown for the first time that rGO-NiPd is a highly active catalyst for hydrolytic dehydrogenation of AB as well as being noble-metal economical and reusable. The NiPd NPs show a composition-dependent catalysis with Ni30Pd70 NPs being the most active at room temperature. The kinetic studies on rGO-Ni30Pd70 catalyzed AB dehydrogenation revealed that it is first order with respect to the catalyst concentration and zeroth order with respect to AB concentration. The apparent activation energy of the rGO-Ni30Pd70 catalyzed AB dehydrogenation is found to be 45 kJ mol1. Furthermore, the rGO-Ni30Pd70 catalyst has also shown an excellent catalytic cycle lifetime by providing a TTON number of 3600 over 35 h and will provide higher at longer reaction times. The rGO-Ni30Pd70 is a promising catalyst in replacing pure noble metals (Pd, Ru, Rh, Pt) for the practical AB hydrolysis as a chemical hydrogen storage tool and to be employed for various reactions.
Acknowledgments This work was supported by the Scientific and Technological Research Council of Turkey (TUBITAK, Project No: 113Z276) and Atatu¨rk University Scientific Research Project Council (Project No: 2013/88).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2014.09.060.
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