Home
Search
Collections
Journals
About
Contact us
My IOPscience
Highly efficient hydrogen generation from methanolysis of ammonia borane on CuPd alloy nanoparticles
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 025401 (http://iopscience.iop.org/0957-4484/26/2/025401) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 194.27.186.18 This content was downloaded on 22/12/2014 at 16:32
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2014) 025401 (9pp)
doi:10.1088/0957-4484/26/2/025401
Highly efficient hydrogen generation from methanolysis of ammonia borane on CuPd alloy nanoparticles Pengyao Li, Zhengli Xiao, Zhaoyan Liu, Jiale Huang, Qingbiao Li and Daohua Sun Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, National Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Xiamen University, Xiamen, 361005, People’s Republic of China E-mail: [email protected] Received 27 September 2014, revised 18 October 2014 Accepted for publication 27 October 2014 Published 17 December 2014 Abstract
A low-cost and facile route has been developed for the synthesis of monodisperse CuPd nanoparticles with tunable composition. (Scanning transmission electron microscopy-energydispersive x-ray spectroscopy) STEM-EDX results verified the structure of the alloy for the obtained nanoparticles. These CuPd nanoparticles supported on carbon were active catalysts for hydrogen generation from the methanolysis of ammonia borane (AB) at room temperature, and their activities were closely related with the compositions. Cu48Pd52 NPs exhibited the highest activity among the tested catalysts. Moreover, their activity can be further improved by thermal annealing at 300 °C under nitrogen flow, with a very high total turnover frequency value of 53.2 min−1. The reusability test indicated that the Cu48Pd52/C catalyst retains 86% of its initial activity and 100% conversion after 8 cycles. The catalyst, which features lost cost and high efficiency, may help move forward the practical application of AB as a sustainable hydrogen storage material. S Online supplementary data available from stacks.iop.org/NANO/26/025401/mmedia Keywords: ammonia borane, methanolysis, alloy catalyst (Some figures may appear in colour only in the online journal) 1. Introduction
methanolysis [7]. Among these three, methanolysis is believed to be advantageous, as the product can be regenerated and no ammonia is released, which is one of the requirements for fuel cell applications [8]. AB can release hydrogen via methanolysis in the presence of a proper catalyst at an ambient temperature; so, the catalyst plays a key role in controlling the hydrogen generation. Noble metals have been well studied on account of their excellent catalytic performance. Ramachandran and Gagare tested various metal salts such as RuCl3, RhCl3, PdCl2 and CoCl2, and found that RuCl3 is the most active for AB methanolysis [5]. However, the difficulty in the separation of the catalyst from the spent fuel has hindered the practical application. The confinement of nanocluster catalysts in porous support materials appears to be an efficient way to
Hydrogen has attracted great interest as a globally accepted clean energy carrier [1]. Although there has been great effort to develop suitable hydrogen storage and releasing materials in the last few decades, the secure storage and effective release of hydrogen are still limiting factors in the ‘Hydrogen Economy’ [2, 3]. Among the new hydrogen materials [2], AB is considered to be the most promising candidate for this purpose [4] owing to its high stability under ambient conditions, nontoxicity and high hydrogen storage density (19.6 wt%). The storage density is greater than the 2015 target of the US Department of Energy (9 wt% hydrogen for a material to be practically applicable) [5, 6]. Hydrogen stored in AB can be released via thermolysis, hydrolysis and 0957-4484/14/025401+09$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 26 (2014) 025401
P Li et al
solve the problem. Saim Özkar prepared Zeolite-Y confined Rh(0) by a two-step procedure: (i) the incorporation of rhodium(III) cations into the zeolite by ion-exchange and (ii) the reduction of rhodium(III) ions within the zeolite cages by sodium borohydride in an aqueous solution [9]. Wang and his co-workers developed a Ru/MMT catalyst by the cationexchange method followed by hydrogen reduction [7]. Usually, noble metal catalysts, such as Ru and Pd, exhibit higher activity than transition metal ones (Ni, Cu). However, the high cost of noble metals can be prohibitive for hydrogen energy applications. It remains challenging to find a catalyst of low cost and high efficiency for hydrogen generation from AB methanolysis. Current efforts have been directed toward bimetallic alloy nanoparticles (NPs), which combine a noble metal with a transition metal. Their catalytic properties were found superior when compared with either component metal [10– 14]. Synergistic effects were achieved with the proper control of the composition, size and structure of the particles. Our recent study [15] also indicated that monodisperse Co48Pd52 NPs were more active than Co-rich or Pd-rich NPs in catalyzing the AB methanolysis reaction. The total turnover frequency (TOF) value reached 27.7 (mol of H2·(mol of catalyst·min)−1). The obtained CoPd NPs were synthesized through the reduction of cobalt acetylacetonate and palladium bromide. In this paper, to further reduce the cost, we chose more abundant Cu in place of Co as the constituents of the bimetallic catalyst. Moreover, the precursors for the two metals were changed to cheap and easily available ones. Meaningfully, the CuPd NPs exhibited enhanced activity (TOF = 53.2 min−1) and desired stability in the methanolysis of AB. Herein, we report the synthesis of CuPd alloy NPs with tunable compositions and the evaluation of their performance in catalyzing the methanolysis of the AB at ambient conditions. We demonstrate that the activity of these CuPd NPs can be enhanced by properly controlling the compositions and the annealing conditions. The CuPd NPs showed high catalytic activity and a prolonged lifetime. These alloy particles can be considered as an efficient and reusable catalyst in the methanolysis of AB to produce hydrogen.
2.2. Synthesis of NPs
In a typical synthesis of CuPd NPs, 0.2 mmol Cu(NO3)2, 0.2 mmol PdCl2 and 0.02 g PVP were mixed with 18 mL of oleylamine under constant nitrogen flow. In order to eliminate the air, the mixture was heated to 110 °C for 0.5 h and then further raised to 230 °C at a heating rate of 4 °C min−1 and kept for 1 h before cooled to room temperature. Afterward, 40.0 mL of isopropanol was added, and the product was separated by centrifugation at 8500 rpm for 8 min. The product was then dispersed in hexane. For comparison, Pd NPs and Cu NPs were also prepared, respectively. The synthesis procedure of Pd NPs was modified from a previous report [16]. Pd(II) acetylacetonate was dissolved in oleylamine at room temperature and then heated to 60 °C when a morpholine borane (MB) complex in oleylamine solution was injected to initiate the nucleation. The mixture solution was further heated to 90 °C and kept for 0.5 h. Then, the obtained product was precipitated out with ethanol, followed by centrifugation; then, it was dispersed in hexane. The particle size was found to be about 4.9 nm. The 3.8 nm Cu NPs were prepared by thermal decomposition of the copper (II) nitrate in the presence of PVP and oleylamine. The procedure was similar to the synthesis of CuPd, except the reduction temperature was set at 250 °C. 2.3. Synthesis of the CuPd/C catalyst
15 mg CuPd NPs were dispersed in 5 mL of hexane and 5 mL of acetone in a 20.0 mL glass vial and mixed with 15 mg of Ketjon carbon. The mixture was sonicated for 2 h to ensure complete deposition of NPs on the carbon support. After evaporation of the solvent under a gentle nitrogen flow, the solid residue was annealed at a certain temperature in constant nitrogen flow for 2 h, and the CuPd/C catalyst was obtained. 2.4. Characterizations
The surface area of the CuPd NPs/C catalysts was measured in a Micromeritics Tristar system (Tristar 3000). Before the measurements, samples were outgassed at room temperature for 3 h. The transmission electron microscopy (TEM) samples were prepared by sonicating CuPd in hexane for 5 min before placing the sample on a carbon-coated gold grid. TEM observations and EDX analysis were performed on an electron microscope (Tecnai F30, FEI; Netherlands) with an accelerating voltage of 300 kV. The content of the metal and the composition in the samples were measured by ICP-MS (Agilent 7700x). The XPS measurements were conducted using an x-ray photoelectric spectrophotometer (Quantum 2000, USA) with Al Kα radiation (40 kV, 30 mA). All of the binding energies were referenced to the C1s peak at 284.8 eV of the surface adventitious carbon. The verification of the products from the catalytic reaction was monitored using 11B NMR spectroscopy. The samples collected at different intervals were transferred into a quartz NMR tube, and the NMR spectra were recorded on a Bruker 400 MHz spectrometer with an operating frequency of 128.15 MHz for 11B.
2. Experimental section 2.1. Chemicals
The copper (II) nitrate hydrate (Cu (NO3)2·3H2O), AR), palladium (II) chloride (PdCl2, AR), polvinylpyrrolidone (PVP, K30), isopropanol (AR), hexane (AR) and methanol (AR) were purchased from Sinopharm Chemical Reagent Co. Ltd (China). The borane-ammonia complex (90%) was purchased from Sigma-Aldrich. The oleylamine (80–90%) was purchased from Aladdin. All of the products were used as received. All of the glassware were cleaned with aqua regia and rinsed several times with de-ionized water. 2
Nanotechnology 26 (2014) 025401
P Li et al
Figure 1. (a) TEM image and size histogram (inset) of CuPd NPs; (b) HRTEM image of a single CuPd NP; (c) distributions of Cu and Pd
components along the cross-sectional line profiles of a single CuPd NP; (d) high-magnification STEM image of CuPd NPs; EDX elemental maps for Cu (e) and Pd (f) concentrations within four individual CuPd NPs. 2.5. Catalyst activity test
The catalytic activity of CuPd NPs/C catalysts in the methanolysis of AB was determined by measuring the rate of hydrogen generation using a water-displacement method. Typically, 10.0 mL of methanol suspension of the catalyst was transferred into the reaction flask (25 mL) placed on a magnetic stirrer, and the temperature was controlled to 25.0 ± 1 °C by circulating water through its jacket from a constant temperature bath. Next, a weighed amount of AB was added into the solution at an 800 rpm stirring rate. The volume of hydrogen gas to be evolved was measured by recording the displacement of the water level, since a graduated glass tube filled with water was connected to the reaction flask. The reaction was ceased when no hydrogen gas generation was observed.
Figure 2. Volume of hydrogen generated from methanolysis of AB
catalyzed by Cu48Pd52/C catalysts annealed at different temperatures for 2 h under constant nitrogen flow. ([AB] = 100 mM, T = 25 ± 1 °C).
3. Results and discussion
[17]. The technique was introduced by Bawendi and coworkers in their report on the synthesis of cadmium chalcogenide nanocrystals [18]. By separating nucleation and growth, monodisperse nanocrystals can be obtained. In our synthesis, the monodisperse CuPd NPs were formed by the reduction of 0.2 mmol Pd (II) chloride and 0.2 mmol Cu (II)
3.1. Synthesis of the monodisperse CuPd NPs
CuPd NPs were fabricated through ‘hot-injection,’ one of the techniques that utilizes homogeneous nucleation to synthesize monodisperse nanocrystals in organic solutions 3
Nanotechnology 26 (2014) 025401
P Li et al
Figure 3. TEM images of Cu48Pd52 NPs without annealing (a) and annealed at the temperatures: (b) 200 °C, (c) 300 °C, (d) 400 °C and (e) 600 °C; (f) distributions of Cu and Pd components along the cross-sectional line profiles of a single Cu48Pd52 NP annealed at 600 °C.
nitrate at 230 °C in the presence of oleylamine and PVP. In the reaction, oleylamine acts as both the solvent and surfactant, and PVP was added as a co-surfactant to stabilize the NPs. The presence of PVP was found to be the key to obtain CuPd NPs with uniform size and higher sphericity compared with the NPs obtained when PVP was absent (figures S1(a)–(b)). Even more importantly, the catalytic activity of PVP-stabled CuPd NPs for AB methanolysis was obviously superior to that of CuPd NPs prepared without PVP (figure S1(c)). ICP-MS analysis revealed the composition of the as-synthesized NPs to be Cu48Pd52. The TEM image in figure 1(a) presents 11.9 nm spherical Cu48Pd52 NPs with a standard deviation of ∼7% in their size. A representative high-resolution TEM (HRTEM) image of a single Cu48Pd52 NP (figure 1(b)) shows the (111) lattice fringe distance of 2.18 Å, which is between the (111) lattice spacing of face-centered cubic (fcc) Cu (2.08 Å) [19] and Pd (2.26 Å) [20]. The STEM-EDX images of the Cu48Pd52 NPs are presented in figures 1(d)–(f). A closer inspection of several as-synthesized NPs suggested that the Cu (figure 1(e)) and Pd (figure 1(f)) atoms were evenly distributed in the entire NPs. Moreover, EDX elemental line scanning on a single NP (figure 1(c)) also verified the alloy structure in the obtained NPs.
3.2. Annealing pretreatment
Shunichi Fukuzumi [21] and Ayman [22] have reported that the catalytic ability for the hydrolysis of AB can be enhanced by a pre-annealing treatment due to the removal of capping agents or the formation of better crystallinity. Our recent report also found that after the annealing pretreatment, the catalyst is more active, and the TOF value of the catalyst also increased from 22.7 to 35.7 [15]. To evaluate the effects of annealing on the catalytic performance of Cu48Pd52 NP, the as-synthesized NPs were first supported on Ketjen carbon and sonificated for 1–2 h to ensure the complete support. After the Cu48Pd52/C NPs were formed, the samples were annealed at varied temperatures (200, 300, 400, 600 °C) for 2 h under constant nitrogen flow. Afterwards, the catalysts were mixed with an AB aqueous solution (1 mmol) at room temperature under magnetic stirring. Meanwhile, the H2 volume generated from the methanolysis of AB was monitored. Figure 2 is a plot of the H2 released versus the time in the presence of 15 mg of Cu48Pd52 NPs annealing at different temperatures. From these curves, we can see that when 1 mmol of AB complex completely reacted, 3 mmol of H2 (∼67 mL) representing the stoichiometric amount is generated. The sooner the methanolysis is completed, the more active the NP catalyst is. For the Cu48Pd52/C NPs without annealing, the 4
Nanotechnology 26 (2014) 025401
P Li et al
Figure 4. XPS spectra of Cu 2p and Pd 3d regions of the Cu48Pd52/C catalyst annealed at (a) 300 °C and (b) 600 °C.
under certain conditions. Annealing at 300 °C under nitrogen flow is optimal for AB methanolysis catalyzed by the CuPd/C catalyst and will be employed in the following experiments. Figure 3 shows the TEM images of the Cu48Pd52 annealed at different temperatures. Compared with the catalyst without annealing (figure 3(a)), the samples pretreated at 200, 300 and 400 °C showed the similar size. Since no obvious morphology change occurred, the enhanced activity is most likely due to the more efficient surfactant removal under the annealing conditions. The surface area of Cu48Pd52/C without annealing and the one annealed at 300 °C were measured to be 312 and 399 m2/g, respectively. Therefore the obvious increase of surface area confirmed the removal of capping agents. When the annealing temperature was set to 600 °C, CuPd NPs began to agglomerate, as shown with the larger size in figure 3(e). The cross-sectional line profiles of a single CuPd NP annealed at 600 °C (figure 3(f)) also indicated that NPs remarkably grew up when compared with the sample without annealing (figure 1(c)), although the structure of the alloy remained. Hence, the increase of particle size led to the notable decrease of catalytic activity. Figure 4 shows the x-ray photoelectron spectroscopy (XPS) spectra of Cu 2p and Pd 3d regions of samples annealed at 300 and 600 °C, respectively. In regard to the Cu 2p spectra of two catalysts, peak of Cu 2p3/2 (932.3 eV) and
Figure 5. Plot of time versus the volume of hydrogen generated from
the methanolysis of AB catalyzed by CuPd/C catalysts with different compositions (NP = 15 mg, [AB] = 100 mM, T = 25 ± 1 °C).
completion time of methanolysis is nearly 4 min. As the calcination temperature increased to 300 °C, the time reduced to 47 s. However, with a further increase of the annealing temperature from 300 °C to 600 °C, the catalytic activity decreased. In particular, at 600 °C, the reaction couldn’t complete within 10 min. Thus, the catalytic performance of the as-prepared CuPd/C can be enhanced by pretreatment 5
Nanotechnology 26 (2014) 025401
P Li et al
Table 1. Activities of Pd-based and Cu-based nanoparticle catalysts in the hydrolysis and methanolysis of AB at room temperature.
catalyst 2 wt% Pd/γ-Al2O3 Pd/zeolite Pd/Ca PSSA-co-MA-Pd Pd/hydroxyapatite PVP stabilized Pda Co35Pd65/C Co48Pd52/Ca Pd@Co/graphene RGO@Pd PdPt cube NPs PdPt spherical NPs Pd/SiO2-CoFe2O4 Cu//γ-Al2O3 Nano Cu@Cu2Oa Nano Cu2Oa Cu/Co3O4 Cu1@Co4 Cu1@Ni4 Graphene-CuCo hybrid [email protected] Cu NPs@SCF [email protected]/RGO Cu-doped titania nanofibers [email protected] [email protected]/graphene Ru1Cu7.5/graphene Cu48Pd52/Ca
Metal/AB ratio (mol mol−1)
Max H2/ AB ratio
Completion time (min)
TOF(mol H2 mol catalyst−1 min−1)
Reference
0.018 0.02 0.02 0.05 0.02 0.005 0.024 0.027 0.02 0.006 0.002 0.002 0.00093 0.018 0.15 0.15 0.05 0.02 0.02 0.02 0.04 0.0031 0.1 0.38 0.092 0.04 0.034 0.072
3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 2.83 1.8 2.2 3.0 3.0 3.0 3.0 3.0 3.0 3.0 2.7 3.0 3.0 3.0 3.0
120 24 80 12 30 27 5.5 4 3.5 19 9 20 18.5 590 / / 3 10 50 16.3 12 50 4 15 3.1 4.85 9 0.783
1.39 6.25 1.9 5 5 22.3 22.7 27.7 42.8 26.3 50.02 22.51 254 0.266 0.16 0.2 18 15 3 9.18 6.25 40 8.36 0.473 10.5 15.46 9.80 53.2
[31] [32] [5] [33] [34] [8] [35] [15] [36] [37] [14] [14] [38] [39] [40] [40] [21] [41] [41] [42] [43] [44] [45] [46] [47] [48] [10] This study
a
Pd-based and Cu-based nanoparticle catalysts employed in the methanolysis of AB. The unmarked representing for the hydrolysis of AB. / not given
room temperature [25, 26]. The experiments indicated that zero-valent Cu and Pd are beneficial for the catalytic activity in the methanolysis of AB.
3.3. Composition effect
Composition also acts as a very important parameter to manipulate catalytic activity in addition to particle dimensions of bimetallic NPs catalysts [27, 28]. CuPd NPs with different compositions were synthesized by changing the ratio of Cu (NO3)2 to PdCl2. An ICP analysis indicated that the as-synthesized NPs was Cu65Pd35, Cu51Pd49, Cu48Pd52, Cu38Pd62 and Cu19Pd81 when the ratio of Cu(NO3)2 to PdCl2 was 4:1, 3:2, 1:1, 2:3 and 1:4, respectively. The typical TEM images (figures S2(B)–(F)) display that the tested CuPd NPs are all 11–13 nm; it is possible to compare their compositiondependent activities and to choose the optimum composition. To clarify if the methanolysis activity of the CuPd NPs was improved by alloying Pd with Cu, 4.9 nm Pd [16] and 3.8 nm Cu NPs were also synthesized for comparison. Figure 5 is the plot of generated H2 versus the time for the AB solution (10 mL, 1 mmol) in the presence of 15 mg NP with different compositions. Obviously, the catalytic performance is closely related to the composition. Under our
11 B NMR spectral stack plot of the methanolysis of AB using the Cu48Pd52 catalyst ([AB] = 100 mM, [CuPd] = 4.8 mM, T = 25 ± 1 °C).
Figure 6.
Cu 2p1/2 (952.1 eV) are exactly the same as the values for zero-valent Cu [23]. Concerning the Pd 3d spectra, the spectra of the sample annealed at 300 °C (figure 4(a)) can be inferred to metallic Pd, while the spectra (figure 4(b)) can be divided into double peaks. The peaks at 335.6 eV (Pd3d5/2) and 340.9 eV (Pd3d3/2) can be safely assigned to metallic Pd, while the peaks at 336.9 eV(Pd3d5/2) and 342.3 eV (Pd3d3/2) showed the presence of Pd(II) [24]. This may be attributed to the fact that the samples were exposed to air between calcination and the XPS analysis, as Pd can react with oxygen at 6
Nanotechnology 26 (2014) 025401
P Li et al
Figure 7. (a) Plot of time versus the volume of hydrogen generated from the methanolysis of AB catalyzed by the Cu48Pd52/C catalyst at different catalyst concentrations and (b) plot of hydrogen generation rate versus the catalyst concentration in the logarithmic scale (rate = mL of H2 s−; [AB] = 100 mM, T = 25 ± 1 °C).
Figure 8. (a) Plot of time versus the volume of hydrogen generated from the methanolysis of AB catalyzed by Cu48Pd52/C catalyst at different AB concentrations and (b) plot of hydrogen generation rate versus the AB concentration in the logarithmic scale (rate = mL of H2 s−1; [CuPd] = 4.8 mM, T = 25 ± 1 °C).
CuPd nanoparticles except Cu19Pd81 exhibited higher activities than pure Pd NPs with a smaller size. The significant enhancement of catalytic activity for the binary Cu-Pd catalysts compared with their mono-counterparts indicates the existence of a synergistic effect between the binary components. Cu48Pd52 displayed the highest activity with the completion time of 47 s for 100 mM AB. Its activity can be evaluated in terms of TOF value, which is 53.2 (mol of H2 (mol of catalyst min)−1). Table 1 summarized the performance of various Cu- and Pd-based catalysts toward the hydrolysis or methanolysis of AB at room temperature. It can be observed that the reaction rate significantly depended on the metal catalysts employed, and the Cu48Pd52/C catalyst is among the most active when considering the TOF value.
Figure 9. TOF of the catalyst and conversion of AB versus the
number of catalytic runs for the Cu48Pd52/C catalyst ([CuPd] = 7.2 mM, [AB] = 100 mM).
3.4. Methanolysis kinetics of the Cu48Pd52/C catalyst
evaluation conditions, Cu NPs were the least active, and the Cu48Pd52 was the most active. The optimal ratio of Cu/Pd is nearly 1/1, which is similar to the original report on Fe0.5Ni0.5 [29] and the Cu-Co catalyst [30]. Here, all of the tested alloy
We selected the Cu48Pd52/C annealed at 300 °C for further study, aiming at a better understanding of the catalytic kinetics of AB methanolysis. First, dehydrogenation of AB in 7
Nanotechnology 26 (2014) 025401
P Li et al
methanol was monitored by 11B NMR spectroscopy. Next, the methanolysis reaction was carried out under the Cu48Pd52 concentration of 2.4, 4.8, 7.2 and 12 mM (representing 5, 10, 15 and 25 mg of Cu48Pd52 NPs, respectively), and the initial AB concentration was kept at 100 mM. Finally, The reactions were performed by keeping the concentration of CuPd NPs constant at 4.8 mM, while the AB concentrations were set at 100, 150, 200 and 250 mM. An acid-base titration showed no ammonia evolution in a detectable amount in the experiments performed in this study, which is very important for fuel-cells applications. Figure 6 presents the spectral stack plot of aliquots of the reaction mixture taken at different time intervals (0, 30, 60 and 90 s). During the course of the reaction, the AB peak at −22.9 ppm damped and eventually disappeared with time. Meanwhile, a new broad resonance at 8.97 ppm assigned to the ammonium tetramethoxy borate appeared and boosted, indicating the reaction occurred as follows [5] NH3 BH3 + 4CH3 OH → NH4 B ( OCH3 )4 + 3H2
the solution indicated the presence of Cu, proving that a small quantity of Cu was detached from the support.
4. Conclusion We have demonstrated that monodisperse CuPd alloy nanoparticles with the mean size of 12 nm could be synthesized with a low-cost process. We further investigated their catalytic performance in the methanolysis of AB at room temperature, and an appreciable catalytic activity was achieved. The catalytic activity depended on the alloy composition, and Cu48Pd52 NPs were the most active. The activity of Cu48Pd52 can be further enhanced by thermal annealing in nitrogen at 300 °C with the TOF value of 53.2 min−1. The kinetic studies on these CuPd NPs revealed that the catalytic methanolysis of AB is first-order with respect to the catalyst concentration and is zero-order with respect to the substrate concentration. Moreover, the reusability of the catalyst was desired, as it retained 86% of the initial activity even after eight catalytic runs. This high catalytic performance of the Cu48Pd52/C catalyst offers an exciting alternative in the pursuit of a practical implementation of AB as a hydrogen storage material for fuel cell applications.
(1)
Figure 7(a) displays the volume of hydrogen generated during the methanolysis of 100 mM AB versus the time in the presence of different concentrations of Cu48Pd52. A linear hydrogen generation rate was observed, and 66.7 mL of hydrogen representing 100% yield in the methanolysis of AB were harvested in all of the cases. We can see that the higher the concentration of NPs is, the sooner the reaction is completed. For the 12 mM Cu48Pd52, the stoichometric amount of H2 was generated within less than 30 s. Figure 7(b) gives the plot of the hydrogen generation rate, determined from the linear portion of the plots in figure 7(a) versus the Cu48Pd52 concentration, which are both in logarithmic scales. The slope of 1.057 indicates that the AB methanolysis reaction is firstorder with respect to the Cu48Pd52 concentration. Figure 8(a) shows the plots of the volume of hydrogen generated versus the time under different AB concentrations. It can be concluded from figure 8(b) that the hydrogen generation rate was practically independent of the AB concentration. Hence, the AB methanolysis reaction is zero-order with respect to the concentration of AB.
Acknowledgments This work was supported by the NSFC projects (Nos. 21206140 and 21036004), the Science and Technology key Program of Fujian Province (No.2013H0044) and the Science and Technology Program of Xiamen, China (No. 3502Z20133006). Appendices Online supplementary data available at stacks.iop.org/ NANO/26/025401/mmedia
References
3.5. Reusability of the Cu48Pd52/C catalyst [1] Schlapbach L and Zuttel A 2001 Nature 414 353–8 [2] Grochala W and Edwards P P 2004 Chem. Rev. 104 1283–316 [3] van den Berg A W and Areán C O 2008 Chem. Commun. 6 668–81 [4] Staubitz A, Robertson A P and Manners I 2010 Chem. Rev. 110 4079–124 [5] Ramachandran P V and Gagare P D 2007 Inorg. Chem. 46 7810–7 [6] Satyapal S, Petrovic J, Read C, Thomas G and Ordaz G 2007 Catal. Today 120 246–56 [7] Dai H-B, Kang X-D and Wang P 2010 Int. J. Hydrog. Energy 35 10317–23 [8] Erdoğan H, Metin Ö and Özkar S 2009 Phys. Chem. Chem. Phys. 11 10519–25 [9] Çalışkan S, Zahmakıran M and Özkar S 2010 Applied Catal. B: Environ. 93 387–94 [10] Cao N, Hu K, Luo W and Cheng G 2014 J. Alloys Compd. 590 241–6 [11] Feng W, Yang L, Cao N, Du C, Dai H, Luo W and Cheng G 2014 Int. J. Hydrog. Energy 39 3371–80
In order to evaluate the reusability of the as-synthesized Cu48Pd52/C catalyst, after the first run of methanolysis of AB (1 mmol) catalyzed by 7.2 mM NPs, the catalyst was kept in the solution, and another equivalent amount of AB was added to the reaction system. The gas generation was monitored, and the same procedure was repeated. As shown in figure 9, the activity of Cu48Pd52/C catalysts decreased after the first cycle; the TOF dropped from 53.2 to 47.5 min−1. After that, the activity almost kept constant. The reusability test showed that the Cu48Pd52/C catalyst retains 86% of its initial activity and 100% conversion after 8 cycles. The TEM image (shown in figure S3) proved there was no significant change either in NP size or morphology; so, the decrease after the 1st run may be attributed to the loss of metal from the surface of carbon [49]. To test our hypothesis, supernatant liquid after the 1st run was filtered to remove the carbon support. The ICP analysis for 8
Nanotechnology 26 (2014) 025401
P Li et al
[30] Li C et al 2013 J. Mater. Chem. A 17 5370–6 [31] Xu Q and Chandra M 2007 J. Alloys Compd. 446 729–32 [32] Rakap M and Özkar S 2010 Int. J. Hydrog. Energy 35 1305–12 [33] Metin Ö, Şahin Ş and Özkar S 2009 Int. J. Hydrog. Energy 34 6304–13 [34] Rakap M and Özkar S 2011 Int. J. Hydrog. Energy 36 7019–27 [35] Sun D, Mazumder V, Metin Ö and Sun S 2011 ACS Nano 5 6458–64 [36] Wang J, Qin Y-L, Liu X and Zhang X-B 2012 J. Mater. Chem. 22 12468–70 [37] Kilic B, Sencanli S and Metin Ö 2012 J. Mol. Catal. A: Chem. 361 104–10 [38] Akbayrak S, Kaya M, Volkan M and Özkar S 2014 Appl. Catal. B-Environ. 147 387–93 [39] Xu Q and Chandra M 2006 J. Power Sources 163 364–70 [40] Kalidindi S B, Sanyal U and Jagirdar B R 2008 Phys. Chem. Chem. Phys. 10 5870–4 [41] Ozay O, Inger E, Aktas N and Sahiner N 2011 Int. J. Hydrog. Energy 36 8209–16 [42] Yan J-M, Wang Z-L, Wang H-L and Jiang Q 2012 J. Mater. Chem. 22 10990–3 [43] Wang H-L, Yan J-M, Wang Z-L and Jiang Q 2012 Int. J. Hydrog. Energy 37 10229–35 [44] Kaya M, Zahmakiran M, Özkar S and Volkan M 2012 ACS Appl. Mater. Interfaces 4 3866–73 [45] Du Y, Cao N, Yang L, Luo W and Cheng G 2013 New J. Chem. 37 3035–42 [46] Yousef A, Barakat N A M and Kim H Y 2013 Appl. Catal. A: Gen. 467 98–106 [47] Qiu F, Dai Y, Li L, Xu C, Huang Y, Chen C, Wang Y, Jiao L and Yuan H 2014 Int. J. Hydrog. Energy 39 436–41 [48] Meng X, Yang L, Cao N, Du C, Hu K, Su J, Luo W and Cheng G 2014 ChemPlusChem 79 325–32 [49] Liang H, Chen G, Desinan S, Rosei R, Rosei F and Ma D 2012 Int. J. Hydrog. Energy 37 17921–7
[12] Cao N, Su J, Luo W and Cheng G 2014 Int. J. Hydrog. Energy 39 426–35 [13] Cao N, Su J, Luo W and Cheng G 2014 Catal. Commun. 43 47–51 [14] Amali A J, Aranishi K, Uchida T and Xu Q 2013 Part. Syst. Charact. 30 888–92 [15] Sun D, Mazumder V, Metin Ö and Sun S 2012 ACS Catal. 2 1290–5 [16] Mazumder V and Sun S 2009 J. Am. Chem. Soc. 131 4588–9 [17] Park J, Joo J, Kwon S G, Jang Y and Hyeon T 2007 Angew. Chem., Int. Ed. 46 4630–60 [18] Murray C B, Norris D J and Bawendi M G 1993 J. Am. Chem. Soc. 115 8706–15 [19] Hansen P L, Wagner J B, Helveg S, Rostrup-Nielsen J R, Clausen B S and Topsøe H 2002 Science 295 2053–5 [20] Dai Z, Bradley J, Joswiak D, Brownlee D, Hill H and Genge M 2002 Nature 418 157–9 [21] Yamada Y, Yano K, Xu Q and Fukuzumi S 2010 J. Phys. Chem. C 114 16456–62 [22] Yousef A, Barakat N A, El-Newehy M and Kim H Y 2012 Int. J. Hydrog. Energy 37 17715–23 [23] Wang H, Huang Y, Tan Z and Hu X 2004 Anal. Chim. Acta 526 13–7 [24] Suo Z, Ma C, Jin M, He T and An L 2008 Catal. Commun. 9 2187–90 [25] Klötzer B, Hayek K, Konvicka C, Lundgren E and Varga P 2001 Surf. Sci. 482 237–42 [26] Ramakrishnan A, Dumbuya K, Ofili J, Steinrück H-P, Gottfried J M and Schwieger W 2011 Applied Clay Science 51 8–14 [27] Zhang Y, Huang W, Habas S E, Kuhn J N, Grass M E, Yamda Y, Yang P and Somorjal G A 2008 J. Phys. Chem. C 112 12092–5 [28] Yang X J, Cheng F Y, Liang J, Tao Z L and Chen J 2011 Int. J. Hydrog. Energy 36 1984–90 [29] Yan J-M, Zhang X-B, Han S, Shioyama H and Xu Q 2009 J. Power Sources 194 478–81
9
Search
Collections
Journals
About
Contact us
My IOPscience
Highly efficient hydrogen generation from methanolysis of ammonia borane on CuPd alloy nanoparticles
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 025401 (http://iopscience.iop.org/0957-4484/26/2/025401) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 194.27.186.18 This content was downloaded on 22/12/2014 at 16:32
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2014) 025401 (9pp)
doi:10.1088/0957-4484/26/2/025401
Highly efficient hydrogen generation from methanolysis of ammonia borane on CuPd alloy nanoparticles Pengyao Li, Zhengli Xiao, Zhaoyan Liu, Jiale Huang, Qingbiao Li and Daohua Sun Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, National Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Xiamen University, Xiamen, 361005, People’s Republic of China E-mail: [email protected] Received 27 September 2014, revised 18 October 2014 Accepted for publication 27 October 2014 Published 17 December 2014 Abstract
A low-cost and facile route has been developed for the synthesis of monodisperse CuPd nanoparticles with tunable composition. (Scanning transmission electron microscopy-energydispersive x-ray spectroscopy) STEM-EDX results verified the structure of the alloy for the obtained nanoparticles. These CuPd nanoparticles supported on carbon were active catalysts for hydrogen generation from the methanolysis of ammonia borane (AB) at room temperature, and their activities were closely related with the compositions. Cu48Pd52 NPs exhibited the highest activity among the tested catalysts. Moreover, their activity can be further improved by thermal annealing at 300 °C under nitrogen flow, with a very high total turnover frequency value of 53.2 min−1. The reusability test indicated that the Cu48Pd52/C catalyst retains 86% of its initial activity and 100% conversion after 8 cycles. The catalyst, which features lost cost and high efficiency, may help move forward the practical application of AB as a sustainable hydrogen storage material. S Online supplementary data available from stacks.iop.org/NANO/26/025401/mmedia Keywords: ammonia borane, methanolysis, alloy catalyst (Some figures may appear in colour only in the online journal) 1. Introduction
methanolysis [7]. Among these three, methanolysis is believed to be advantageous, as the product can be regenerated and no ammonia is released, which is one of the requirements for fuel cell applications [8]. AB can release hydrogen via methanolysis in the presence of a proper catalyst at an ambient temperature; so, the catalyst plays a key role in controlling the hydrogen generation. Noble metals have been well studied on account of their excellent catalytic performance. Ramachandran and Gagare tested various metal salts such as RuCl3, RhCl3, PdCl2 and CoCl2, and found that RuCl3 is the most active for AB methanolysis [5]. However, the difficulty in the separation of the catalyst from the spent fuel has hindered the practical application. The confinement of nanocluster catalysts in porous support materials appears to be an efficient way to
Hydrogen has attracted great interest as a globally accepted clean energy carrier [1]. Although there has been great effort to develop suitable hydrogen storage and releasing materials in the last few decades, the secure storage and effective release of hydrogen are still limiting factors in the ‘Hydrogen Economy’ [2, 3]. Among the new hydrogen materials [2], AB is considered to be the most promising candidate for this purpose [4] owing to its high stability under ambient conditions, nontoxicity and high hydrogen storage density (19.6 wt%). The storage density is greater than the 2015 target of the US Department of Energy (9 wt% hydrogen for a material to be practically applicable) [5, 6]. Hydrogen stored in AB can be released via thermolysis, hydrolysis and 0957-4484/14/025401+09$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 26 (2014) 025401
P Li et al
solve the problem. Saim Özkar prepared Zeolite-Y confined Rh(0) by a two-step procedure: (i) the incorporation of rhodium(III) cations into the zeolite by ion-exchange and (ii) the reduction of rhodium(III) ions within the zeolite cages by sodium borohydride in an aqueous solution [9]. Wang and his co-workers developed a Ru/MMT catalyst by the cationexchange method followed by hydrogen reduction [7]. Usually, noble metal catalysts, such as Ru and Pd, exhibit higher activity than transition metal ones (Ni, Cu). However, the high cost of noble metals can be prohibitive for hydrogen energy applications. It remains challenging to find a catalyst of low cost and high efficiency for hydrogen generation from AB methanolysis. Current efforts have been directed toward bimetallic alloy nanoparticles (NPs), which combine a noble metal with a transition metal. Their catalytic properties were found superior when compared with either component metal [10– 14]. Synergistic effects were achieved with the proper control of the composition, size and structure of the particles. Our recent study [15] also indicated that monodisperse Co48Pd52 NPs were more active than Co-rich or Pd-rich NPs in catalyzing the AB methanolysis reaction. The total turnover frequency (TOF) value reached 27.7 (mol of H2·(mol of catalyst·min)−1). The obtained CoPd NPs were synthesized through the reduction of cobalt acetylacetonate and palladium bromide. In this paper, to further reduce the cost, we chose more abundant Cu in place of Co as the constituents of the bimetallic catalyst. Moreover, the precursors for the two metals were changed to cheap and easily available ones. Meaningfully, the CuPd NPs exhibited enhanced activity (TOF = 53.2 min−1) and desired stability in the methanolysis of AB. Herein, we report the synthesis of CuPd alloy NPs with tunable compositions and the evaluation of their performance in catalyzing the methanolysis of the AB at ambient conditions. We demonstrate that the activity of these CuPd NPs can be enhanced by properly controlling the compositions and the annealing conditions. The CuPd NPs showed high catalytic activity and a prolonged lifetime. These alloy particles can be considered as an efficient and reusable catalyst in the methanolysis of AB to produce hydrogen.
2.2. Synthesis of NPs
In a typical synthesis of CuPd NPs, 0.2 mmol Cu(NO3)2, 0.2 mmol PdCl2 and 0.02 g PVP were mixed with 18 mL of oleylamine under constant nitrogen flow. In order to eliminate the air, the mixture was heated to 110 °C for 0.5 h and then further raised to 230 °C at a heating rate of 4 °C min−1 and kept for 1 h before cooled to room temperature. Afterward, 40.0 mL of isopropanol was added, and the product was separated by centrifugation at 8500 rpm for 8 min. The product was then dispersed in hexane. For comparison, Pd NPs and Cu NPs were also prepared, respectively. The synthesis procedure of Pd NPs was modified from a previous report [16]. Pd(II) acetylacetonate was dissolved in oleylamine at room temperature and then heated to 60 °C when a morpholine borane (MB) complex in oleylamine solution was injected to initiate the nucleation. The mixture solution was further heated to 90 °C and kept for 0.5 h. Then, the obtained product was precipitated out with ethanol, followed by centrifugation; then, it was dispersed in hexane. The particle size was found to be about 4.9 nm. The 3.8 nm Cu NPs were prepared by thermal decomposition of the copper (II) nitrate in the presence of PVP and oleylamine. The procedure was similar to the synthesis of CuPd, except the reduction temperature was set at 250 °C. 2.3. Synthesis of the CuPd/C catalyst
15 mg CuPd NPs were dispersed in 5 mL of hexane and 5 mL of acetone in a 20.0 mL glass vial and mixed with 15 mg of Ketjon carbon. The mixture was sonicated for 2 h to ensure complete deposition of NPs on the carbon support. After evaporation of the solvent under a gentle nitrogen flow, the solid residue was annealed at a certain temperature in constant nitrogen flow for 2 h, and the CuPd/C catalyst was obtained. 2.4. Characterizations
The surface area of the CuPd NPs/C catalysts was measured in a Micromeritics Tristar system (Tristar 3000). Before the measurements, samples were outgassed at room temperature for 3 h. The transmission electron microscopy (TEM) samples were prepared by sonicating CuPd in hexane for 5 min before placing the sample on a carbon-coated gold grid. TEM observations and EDX analysis were performed on an electron microscope (Tecnai F30, FEI; Netherlands) with an accelerating voltage of 300 kV. The content of the metal and the composition in the samples were measured by ICP-MS (Agilent 7700x). The XPS measurements were conducted using an x-ray photoelectric spectrophotometer (Quantum 2000, USA) with Al Kα radiation (40 kV, 30 mA). All of the binding energies were referenced to the C1s peak at 284.8 eV of the surface adventitious carbon. The verification of the products from the catalytic reaction was monitored using 11B NMR spectroscopy. The samples collected at different intervals were transferred into a quartz NMR tube, and the NMR spectra were recorded on a Bruker 400 MHz spectrometer with an operating frequency of 128.15 MHz for 11B.
2. Experimental section 2.1. Chemicals
The copper (II) nitrate hydrate (Cu (NO3)2·3H2O), AR), palladium (II) chloride (PdCl2, AR), polvinylpyrrolidone (PVP, K30), isopropanol (AR), hexane (AR) and methanol (AR) were purchased from Sinopharm Chemical Reagent Co. Ltd (China). The borane-ammonia complex (90%) was purchased from Sigma-Aldrich. The oleylamine (80–90%) was purchased from Aladdin. All of the products were used as received. All of the glassware were cleaned with aqua regia and rinsed several times with de-ionized water. 2
Nanotechnology 26 (2014) 025401
P Li et al
Figure 1. (a) TEM image and size histogram (inset) of CuPd NPs; (b) HRTEM image of a single CuPd NP; (c) distributions of Cu and Pd
components along the cross-sectional line profiles of a single CuPd NP; (d) high-magnification STEM image of CuPd NPs; EDX elemental maps for Cu (e) and Pd (f) concentrations within four individual CuPd NPs. 2.5. Catalyst activity test
The catalytic activity of CuPd NPs/C catalysts in the methanolysis of AB was determined by measuring the rate of hydrogen generation using a water-displacement method. Typically, 10.0 mL of methanol suspension of the catalyst was transferred into the reaction flask (25 mL) placed on a magnetic stirrer, and the temperature was controlled to 25.0 ± 1 °C by circulating water through its jacket from a constant temperature bath. Next, a weighed amount of AB was added into the solution at an 800 rpm stirring rate. The volume of hydrogen gas to be evolved was measured by recording the displacement of the water level, since a graduated glass tube filled with water was connected to the reaction flask. The reaction was ceased when no hydrogen gas generation was observed.
Figure 2. Volume of hydrogen generated from methanolysis of AB
catalyzed by Cu48Pd52/C catalysts annealed at different temperatures for 2 h under constant nitrogen flow. ([AB] = 100 mM, T = 25 ± 1 °C).
3. Results and discussion
[17]. The technique was introduced by Bawendi and coworkers in their report on the synthesis of cadmium chalcogenide nanocrystals [18]. By separating nucleation and growth, monodisperse nanocrystals can be obtained. In our synthesis, the monodisperse CuPd NPs were formed by the reduction of 0.2 mmol Pd (II) chloride and 0.2 mmol Cu (II)
3.1. Synthesis of the monodisperse CuPd NPs
CuPd NPs were fabricated through ‘hot-injection,’ one of the techniques that utilizes homogeneous nucleation to synthesize monodisperse nanocrystals in organic solutions 3
Nanotechnology 26 (2014) 025401
P Li et al
Figure 3. TEM images of Cu48Pd52 NPs without annealing (a) and annealed at the temperatures: (b) 200 °C, (c) 300 °C, (d) 400 °C and (e) 600 °C; (f) distributions of Cu and Pd components along the cross-sectional line profiles of a single Cu48Pd52 NP annealed at 600 °C.
nitrate at 230 °C in the presence of oleylamine and PVP. In the reaction, oleylamine acts as both the solvent and surfactant, and PVP was added as a co-surfactant to stabilize the NPs. The presence of PVP was found to be the key to obtain CuPd NPs with uniform size and higher sphericity compared with the NPs obtained when PVP was absent (figures S1(a)–(b)). Even more importantly, the catalytic activity of PVP-stabled CuPd NPs for AB methanolysis was obviously superior to that of CuPd NPs prepared without PVP (figure S1(c)). ICP-MS analysis revealed the composition of the as-synthesized NPs to be Cu48Pd52. The TEM image in figure 1(a) presents 11.9 nm spherical Cu48Pd52 NPs with a standard deviation of ∼7% in their size. A representative high-resolution TEM (HRTEM) image of a single Cu48Pd52 NP (figure 1(b)) shows the (111) lattice fringe distance of 2.18 Å, which is between the (111) lattice spacing of face-centered cubic (fcc) Cu (2.08 Å) [19] and Pd (2.26 Å) [20]. The STEM-EDX images of the Cu48Pd52 NPs are presented in figures 1(d)–(f). A closer inspection of several as-synthesized NPs suggested that the Cu (figure 1(e)) and Pd (figure 1(f)) atoms were evenly distributed in the entire NPs. Moreover, EDX elemental line scanning on a single NP (figure 1(c)) also verified the alloy structure in the obtained NPs.
3.2. Annealing pretreatment
Shunichi Fukuzumi [21] and Ayman [22] have reported that the catalytic ability for the hydrolysis of AB can be enhanced by a pre-annealing treatment due to the removal of capping agents or the formation of better crystallinity. Our recent report also found that after the annealing pretreatment, the catalyst is more active, and the TOF value of the catalyst also increased from 22.7 to 35.7 [15]. To evaluate the effects of annealing on the catalytic performance of Cu48Pd52 NP, the as-synthesized NPs were first supported on Ketjen carbon and sonificated for 1–2 h to ensure the complete support. After the Cu48Pd52/C NPs were formed, the samples were annealed at varied temperatures (200, 300, 400, 600 °C) for 2 h under constant nitrogen flow. Afterwards, the catalysts were mixed with an AB aqueous solution (1 mmol) at room temperature under magnetic stirring. Meanwhile, the H2 volume generated from the methanolysis of AB was monitored. Figure 2 is a plot of the H2 released versus the time in the presence of 15 mg of Cu48Pd52 NPs annealing at different temperatures. From these curves, we can see that when 1 mmol of AB complex completely reacted, 3 mmol of H2 (∼67 mL) representing the stoichiometric amount is generated. The sooner the methanolysis is completed, the more active the NP catalyst is. For the Cu48Pd52/C NPs without annealing, the 4
Nanotechnology 26 (2014) 025401
P Li et al
Figure 4. XPS spectra of Cu 2p and Pd 3d regions of the Cu48Pd52/C catalyst annealed at (a) 300 °C and (b) 600 °C.
under certain conditions. Annealing at 300 °C under nitrogen flow is optimal for AB methanolysis catalyzed by the CuPd/C catalyst and will be employed in the following experiments. Figure 3 shows the TEM images of the Cu48Pd52 annealed at different temperatures. Compared with the catalyst without annealing (figure 3(a)), the samples pretreated at 200, 300 and 400 °C showed the similar size. Since no obvious morphology change occurred, the enhanced activity is most likely due to the more efficient surfactant removal under the annealing conditions. The surface area of Cu48Pd52/C without annealing and the one annealed at 300 °C were measured to be 312 and 399 m2/g, respectively. Therefore the obvious increase of surface area confirmed the removal of capping agents. When the annealing temperature was set to 600 °C, CuPd NPs began to agglomerate, as shown with the larger size in figure 3(e). The cross-sectional line profiles of a single CuPd NP annealed at 600 °C (figure 3(f)) also indicated that NPs remarkably grew up when compared with the sample without annealing (figure 1(c)), although the structure of the alloy remained. Hence, the increase of particle size led to the notable decrease of catalytic activity. Figure 4 shows the x-ray photoelectron spectroscopy (XPS) spectra of Cu 2p and Pd 3d regions of samples annealed at 300 and 600 °C, respectively. In regard to the Cu 2p spectra of two catalysts, peak of Cu 2p3/2 (932.3 eV) and
Figure 5. Plot of time versus the volume of hydrogen generated from
the methanolysis of AB catalyzed by CuPd/C catalysts with different compositions (NP = 15 mg, [AB] = 100 mM, T = 25 ± 1 °C).
completion time of methanolysis is nearly 4 min. As the calcination temperature increased to 300 °C, the time reduced to 47 s. However, with a further increase of the annealing temperature from 300 °C to 600 °C, the catalytic activity decreased. In particular, at 600 °C, the reaction couldn’t complete within 10 min. Thus, the catalytic performance of the as-prepared CuPd/C can be enhanced by pretreatment 5
Nanotechnology 26 (2014) 025401
P Li et al
Table 1. Activities of Pd-based and Cu-based nanoparticle catalysts in the hydrolysis and methanolysis of AB at room temperature.
catalyst 2 wt% Pd/γ-Al2O3 Pd/zeolite Pd/Ca PSSA-co-MA-Pd Pd/hydroxyapatite PVP stabilized Pda Co35Pd65/C Co48Pd52/Ca Pd@Co/graphene RGO@Pd PdPt cube NPs PdPt spherical NPs Pd/SiO2-CoFe2O4 Cu//γ-Al2O3 Nano Cu@Cu2Oa Nano Cu2Oa Cu/Co3O4 Cu1@Co4 Cu1@Ni4 Graphene-CuCo hybrid [email protected] Cu NPs@SCF [email protected]/RGO Cu-doped titania nanofibers [email protected] [email protected]/graphene Ru1Cu7.5/graphene Cu48Pd52/Ca
Metal/AB ratio (mol mol−1)
Max H2/ AB ratio
Completion time (min)
TOF(mol H2 mol catalyst−1 min−1)
Reference
0.018 0.02 0.02 0.05 0.02 0.005 0.024 0.027 0.02 0.006 0.002 0.002 0.00093 0.018 0.15 0.15 0.05 0.02 0.02 0.02 0.04 0.0031 0.1 0.38 0.092 0.04 0.034 0.072
3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 2.83 1.8 2.2 3.0 3.0 3.0 3.0 3.0 3.0 3.0 2.7 3.0 3.0 3.0 3.0
120 24 80 12 30 27 5.5 4 3.5 19 9 20 18.5 590 / / 3 10 50 16.3 12 50 4 15 3.1 4.85 9 0.783
1.39 6.25 1.9 5 5 22.3 22.7 27.7 42.8 26.3 50.02 22.51 254 0.266 0.16 0.2 18 15 3 9.18 6.25 40 8.36 0.473 10.5 15.46 9.80 53.2
[31] [32] [5] [33] [34] [8] [35] [15] [36] [37] [14] [14] [38] [39] [40] [40] [21] [41] [41] [42] [43] [44] [45] [46] [47] [48] [10] This study
a
Pd-based and Cu-based nanoparticle catalysts employed in the methanolysis of AB. The unmarked representing for the hydrolysis of AB. / not given
room temperature [25, 26]. The experiments indicated that zero-valent Cu and Pd are beneficial for the catalytic activity in the methanolysis of AB.
3.3. Composition effect
Composition also acts as a very important parameter to manipulate catalytic activity in addition to particle dimensions of bimetallic NPs catalysts [27, 28]. CuPd NPs with different compositions were synthesized by changing the ratio of Cu (NO3)2 to PdCl2. An ICP analysis indicated that the as-synthesized NPs was Cu65Pd35, Cu51Pd49, Cu48Pd52, Cu38Pd62 and Cu19Pd81 when the ratio of Cu(NO3)2 to PdCl2 was 4:1, 3:2, 1:1, 2:3 and 1:4, respectively. The typical TEM images (figures S2(B)–(F)) display that the tested CuPd NPs are all 11–13 nm; it is possible to compare their compositiondependent activities and to choose the optimum composition. To clarify if the methanolysis activity of the CuPd NPs was improved by alloying Pd with Cu, 4.9 nm Pd [16] and 3.8 nm Cu NPs were also synthesized for comparison. Figure 5 is the plot of generated H2 versus the time for the AB solution (10 mL, 1 mmol) in the presence of 15 mg NP with different compositions. Obviously, the catalytic performance is closely related to the composition. Under our
11 B NMR spectral stack plot of the methanolysis of AB using the Cu48Pd52 catalyst ([AB] = 100 mM, [CuPd] = 4.8 mM, T = 25 ± 1 °C).
Figure 6.
Cu 2p1/2 (952.1 eV) are exactly the same as the values for zero-valent Cu [23]. Concerning the Pd 3d spectra, the spectra of the sample annealed at 300 °C (figure 4(a)) can be inferred to metallic Pd, while the spectra (figure 4(b)) can be divided into double peaks. The peaks at 335.6 eV (Pd3d5/2) and 340.9 eV (Pd3d3/2) can be safely assigned to metallic Pd, while the peaks at 336.9 eV(Pd3d5/2) and 342.3 eV (Pd3d3/2) showed the presence of Pd(II) [24]. This may be attributed to the fact that the samples were exposed to air between calcination and the XPS analysis, as Pd can react with oxygen at 6
Nanotechnology 26 (2014) 025401
P Li et al
Figure 7. (a) Plot of time versus the volume of hydrogen generated from the methanolysis of AB catalyzed by the Cu48Pd52/C catalyst at different catalyst concentrations and (b) plot of hydrogen generation rate versus the catalyst concentration in the logarithmic scale (rate = mL of H2 s−; [AB] = 100 mM, T = 25 ± 1 °C).
Figure 8. (a) Plot of time versus the volume of hydrogen generated from the methanolysis of AB catalyzed by Cu48Pd52/C catalyst at different AB concentrations and (b) plot of hydrogen generation rate versus the AB concentration in the logarithmic scale (rate = mL of H2 s−1; [CuPd] = 4.8 mM, T = 25 ± 1 °C).
CuPd nanoparticles except Cu19Pd81 exhibited higher activities than pure Pd NPs with a smaller size. The significant enhancement of catalytic activity for the binary Cu-Pd catalysts compared with their mono-counterparts indicates the existence of a synergistic effect between the binary components. Cu48Pd52 displayed the highest activity with the completion time of 47 s for 100 mM AB. Its activity can be evaluated in terms of TOF value, which is 53.2 (mol of H2 (mol of catalyst min)−1). Table 1 summarized the performance of various Cu- and Pd-based catalysts toward the hydrolysis or methanolysis of AB at room temperature. It can be observed that the reaction rate significantly depended on the metal catalysts employed, and the Cu48Pd52/C catalyst is among the most active when considering the TOF value.
Figure 9. TOF of the catalyst and conversion of AB versus the
number of catalytic runs for the Cu48Pd52/C catalyst ([CuPd] = 7.2 mM, [AB] = 100 mM).
3.4. Methanolysis kinetics of the Cu48Pd52/C catalyst
evaluation conditions, Cu NPs were the least active, and the Cu48Pd52 was the most active. The optimal ratio of Cu/Pd is nearly 1/1, which is similar to the original report on Fe0.5Ni0.5 [29] and the Cu-Co catalyst [30]. Here, all of the tested alloy
We selected the Cu48Pd52/C annealed at 300 °C for further study, aiming at a better understanding of the catalytic kinetics of AB methanolysis. First, dehydrogenation of AB in 7
Nanotechnology 26 (2014) 025401
P Li et al
methanol was monitored by 11B NMR spectroscopy. Next, the methanolysis reaction was carried out under the Cu48Pd52 concentration of 2.4, 4.8, 7.2 and 12 mM (representing 5, 10, 15 and 25 mg of Cu48Pd52 NPs, respectively), and the initial AB concentration was kept at 100 mM. Finally, The reactions were performed by keeping the concentration of CuPd NPs constant at 4.8 mM, while the AB concentrations were set at 100, 150, 200 and 250 mM. An acid-base titration showed no ammonia evolution in a detectable amount in the experiments performed in this study, which is very important for fuel-cells applications. Figure 6 presents the spectral stack plot of aliquots of the reaction mixture taken at different time intervals (0, 30, 60 and 90 s). During the course of the reaction, the AB peak at −22.9 ppm damped and eventually disappeared with time. Meanwhile, a new broad resonance at 8.97 ppm assigned to the ammonium tetramethoxy borate appeared and boosted, indicating the reaction occurred as follows [5] NH3 BH3 + 4CH3 OH → NH4 B ( OCH3 )4 + 3H2
the solution indicated the presence of Cu, proving that a small quantity of Cu was detached from the support.
4. Conclusion We have demonstrated that monodisperse CuPd alloy nanoparticles with the mean size of 12 nm could be synthesized with a low-cost process. We further investigated their catalytic performance in the methanolysis of AB at room temperature, and an appreciable catalytic activity was achieved. The catalytic activity depended on the alloy composition, and Cu48Pd52 NPs were the most active. The activity of Cu48Pd52 can be further enhanced by thermal annealing in nitrogen at 300 °C with the TOF value of 53.2 min−1. The kinetic studies on these CuPd NPs revealed that the catalytic methanolysis of AB is first-order with respect to the catalyst concentration and is zero-order with respect to the substrate concentration. Moreover, the reusability of the catalyst was desired, as it retained 86% of the initial activity even after eight catalytic runs. This high catalytic performance of the Cu48Pd52/C catalyst offers an exciting alternative in the pursuit of a practical implementation of AB as a hydrogen storage material for fuel cell applications.
(1)
Figure 7(a) displays the volume of hydrogen generated during the methanolysis of 100 mM AB versus the time in the presence of different concentrations of Cu48Pd52. A linear hydrogen generation rate was observed, and 66.7 mL of hydrogen representing 100% yield in the methanolysis of AB were harvested in all of the cases. We can see that the higher the concentration of NPs is, the sooner the reaction is completed. For the 12 mM Cu48Pd52, the stoichometric amount of H2 was generated within less than 30 s. Figure 7(b) gives the plot of the hydrogen generation rate, determined from the linear portion of the plots in figure 7(a) versus the Cu48Pd52 concentration, which are both in logarithmic scales. The slope of 1.057 indicates that the AB methanolysis reaction is firstorder with respect to the Cu48Pd52 concentration. Figure 8(a) shows the plots of the volume of hydrogen generated versus the time under different AB concentrations. It can be concluded from figure 8(b) that the hydrogen generation rate was practically independent of the AB concentration. Hence, the AB methanolysis reaction is zero-order with respect to the concentration of AB.
Acknowledgments This work was supported by the NSFC projects (Nos. 21206140 and 21036004), the Science and Technology key Program of Fujian Province (No.2013H0044) and the Science and Technology Program of Xiamen, China (No. 3502Z20133006). Appendices Online supplementary data available at stacks.iop.org/ NANO/26/025401/mmedia
References
3.5. Reusability of the Cu48Pd52/C catalyst [1] Schlapbach L and Zuttel A 2001 Nature 414 353–8 [2] Grochala W and Edwards P P 2004 Chem. Rev. 104 1283–316 [3] van den Berg A W and Areán C O 2008 Chem. Commun. 6 668–81 [4] Staubitz A, Robertson A P and Manners I 2010 Chem. Rev. 110 4079–124 [5] Ramachandran P V and Gagare P D 2007 Inorg. Chem. 46 7810–7 [6] Satyapal S, Petrovic J, Read C, Thomas G and Ordaz G 2007 Catal. Today 120 246–56 [7] Dai H-B, Kang X-D and Wang P 2010 Int. J. Hydrog. Energy 35 10317–23 [8] Erdoğan H, Metin Ö and Özkar S 2009 Phys. Chem. Chem. Phys. 11 10519–25 [9] Çalışkan S, Zahmakıran M and Özkar S 2010 Applied Catal. B: Environ. 93 387–94 [10] Cao N, Hu K, Luo W and Cheng G 2014 J. Alloys Compd. 590 241–6 [11] Feng W, Yang L, Cao N, Du C, Dai H, Luo W and Cheng G 2014 Int. J. Hydrog. Energy 39 3371–80
In order to evaluate the reusability of the as-synthesized Cu48Pd52/C catalyst, after the first run of methanolysis of AB (1 mmol) catalyzed by 7.2 mM NPs, the catalyst was kept in the solution, and another equivalent amount of AB was added to the reaction system. The gas generation was monitored, and the same procedure was repeated. As shown in figure 9, the activity of Cu48Pd52/C catalysts decreased after the first cycle; the TOF dropped from 53.2 to 47.5 min−1. After that, the activity almost kept constant. The reusability test showed that the Cu48Pd52/C catalyst retains 86% of its initial activity and 100% conversion after 8 cycles. The TEM image (shown in figure S3) proved there was no significant change either in NP size or morphology; so, the decrease after the 1st run may be attributed to the loss of metal from the surface of carbon [49]. To test our hypothesis, supernatant liquid after the 1st run was filtered to remove the carbon support. The ICP analysis for 8
Nanotechnology 26 (2014) 025401
P Li et al
[30] Li C et al 2013 J. Mater. Chem. A 17 5370–6 [31] Xu Q and Chandra M 2007 J. Alloys Compd. 446 729–32 [32] Rakap M and Özkar S 2010 Int. J. Hydrog. Energy 35 1305–12 [33] Metin Ö, Şahin Ş and Özkar S 2009 Int. J. Hydrog. Energy 34 6304–13 [34] Rakap M and Özkar S 2011 Int. J. Hydrog. Energy 36 7019–27 [35] Sun D, Mazumder V, Metin Ö and Sun S 2011 ACS Nano 5 6458–64 [36] Wang J, Qin Y-L, Liu X and Zhang X-B 2012 J. Mater. Chem. 22 12468–70 [37] Kilic B, Sencanli S and Metin Ö 2012 J. Mol. Catal. A: Chem. 361 104–10 [38] Akbayrak S, Kaya M, Volkan M and Özkar S 2014 Appl. Catal. B-Environ. 147 387–93 [39] Xu Q and Chandra M 2006 J. Power Sources 163 364–70 [40] Kalidindi S B, Sanyal U and Jagirdar B R 2008 Phys. Chem. Chem. Phys. 10 5870–4 [41] Ozay O, Inger E, Aktas N and Sahiner N 2011 Int. J. Hydrog. Energy 36 8209–16 [42] Yan J-M, Wang Z-L, Wang H-L and Jiang Q 2012 J. Mater. Chem. 22 10990–3 [43] Wang H-L, Yan J-M, Wang Z-L and Jiang Q 2012 Int. J. Hydrog. Energy 37 10229–35 [44] Kaya M, Zahmakiran M, Özkar S and Volkan M 2012 ACS Appl. Mater. Interfaces 4 3866–73 [45] Du Y, Cao N, Yang L, Luo W and Cheng G 2013 New J. Chem. 37 3035–42 [46] Yousef A, Barakat N A M and Kim H Y 2013 Appl. Catal. A: Gen. 467 98–106 [47] Qiu F, Dai Y, Li L, Xu C, Huang Y, Chen C, Wang Y, Jiao L and Yuan H 2014 Int. J. Hydrog. Energy 39 436–41 [48] Meng X, Yang L, Cao N, Du C, Hu K, Su J, Luo W and Cheng G 2014 ChemPlusChem 79 325–32 [49] Liang H, Chen G, Desinan S, Rosei R, Rosei F and Ma D 2012 Int. J. Hydrog. Energy 37 17921–7
[12] Cao N, Su J, Luo W and Cheng G 2014 Int. J. Hydrog. Energy 39 426–35 [13] Cao N, Su J, Luo W and Cheng G 2014 Catal. Commun. 43 47–51 [14] Amali A J, Aranishi K, Uchida T and Xu Q 2013 Part. Syst. Charact. 30 888–92 [15] Sun D, Mazumder V, Metin Ö and Sun S 2012 ACS Catal. 2 1290–5 [16] Mazumder V and Sun S 2009 J. Am. Chem. Soc. 131 4588–9 [17] Park J, Joo J, Kwon S G, Jang Y and Hyeon T 2007 Angew. Chem., Int. Ed. 46 4630–60 [18] Murray C B, Norris D J and Bawendi M G 1993 J. Am. Chem. Soc. 115 8706–15 [19] Hansen P L, Wagner J B, Helveg S, Rostrup-Nielsen J R, Clausen B S and Topsøe H 2002 Science 295 2053–5 [20] Dai Z, Bradley J, Joswiak D, Brownlee D, Hill H and Genge M 2002 Nature 418 157–9 [21] Yamada Y, Yano K, Xu Q and Fukuzumi S 2010 J. Phys. Chem. C 114 16456–62 [22] Yousef A, Barakat N A, El-Newehy M and Kim H Y 2012 Int. J. Hydrog. Energy 37 17715–23 [23] Wang H, Huang Y, Tan Z and Hu X 2004 Anal. Chim. Acta 526 13–7 [24] Suo Z, Ma C, Jin M, He T and An L 2008 Catal. Commun. 9 2187–90 [25] Klötzer B, Hayek K, Konvicka C, Lundgren E and Varga P 2001 Surf. Sci. 482 237–42 [26] Ramakrishnan A, Dumbuya K, Ofili J, Steinrück H-P, Gottfried J M and Schwieger W 2011 Applied Clay Science 51 8–14 [27] Zhang Y, Huang W, Habas S E, Kuhn J N, Grass M E, Yamda Y, Yang P and Somorjal G A 2008 J. Phys. Chem. C 112 12092–5 [28] Yang X J, Cheng F Y, Liang J, Tao Z L and Chen J 2011 Int. J. Hydrog. Energy 36 1984–90 [29] Yan J-M, Zhang X-B, Han S, Shioyama H and Xu Q 2009 J. Power Sources 194 478–81
9
