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PVP-stabilized Ru–Rh nanoparticles as highly efficient catalysts for hydrogen generation from hydrolysis of ammonia borane
Journal of Alloys and Compounds 649 (2015) 1025e1030
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
PVP-stabilized RueRh nanoparticles as highly efficient catalysts for hydrogen generation from hydrolysis of ammonia borane Murat Rakap Maritime Faculty, Yuzuncu Yil University, 65080, Van, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 May 2015 Received in revised form 23 July 2015 Accepted 27 July 2015 Available online 29 July 2015
Herein, the utilization of poly(N-vinyl-2-pyrrolidone)-protected rutheniumerhodium nanoparticles (3.4 ± 1.4 nm) as highly efficient catalysts in the hydrolysis of ammonia borane for hydrogen generation is reported. They are prepared by co-reduction of ruthenium and rhodium metal ions in ethanol/water mixture by an alcohol reduction method and characterized by transmission electron microscopy-energy dispersive X-ray spectroscopy, ultravioletevisible spectroscopy, and X-ray photoelectron spectroscopy. They are durable and highly efficient catalysts for hydrogen generation from the hydrolysis of ammonia borane even at very low concentrations and temperature, providing average turnover frequency of 386 mol H2 (mol cat)1 min1 and maximum hydrogen generation rate of 10,680 L H2 min1 (mol cat)1. Poly(N-vinyl-2-pyrrolidone)-protected rutheniumerhodium nanoparticles also provide activation energy of 47.4 ± 2.1 kJ/mol for the hydrolysis of ammonia borane. © 2015 Elsevier B.V. All rights reserved.
Keywords: Ruthenium Rhodium Nanoparticle Ammonia borane Hydrogen
1. Introduction Employment of chemical hydrides as efficient and safe hydrogen storage materials is one of the most promising approaches [1]. For this purpose, ammonia borane (H3NBH3, AB) has recently been employed as a solid hydrogen storage material due to its high hydrogen content (19.6 wt %), high solubility and stability in water at room temperature [2,3]. AB is able to release hydrogen upon hydrolysis at room temperature in the presence of suitable catalysts according to Eq. (1): catalyst
H3 NBH3 ðaqÞ þ 2H2 OðlÞ !NHþ 4 ðaqÞ þ BO2 ðaqÞ þ 3H2 ðgÞ
(1) Recently, different kinds of bimetallic nanoparticle-type catalysts have been tested for hydrogen generation from the hydrolysis of AB since the addition of second element to the monometallic nanoparticles definitely improves the catalytic properties. Ni@Ru coreeshell nanoparticles [4], NieRu alloy nanoparticles [5], RuCo and RuCu on ɤ-Al2O3 [6], RuCu on graphene [7], Ru@Ni coreeshell nanoparticles [8], CueNi on MCM-41 [9], Ru@Co on graphene [10], CoNi@rGO [11], CuCo@MOF nanoparticles [12], Ni/Pt hollow nanospheres [13], Ag/Pd@nanofiber nanoparticles [14], Pt-M
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.jallcom.2015.07.249 0925-8388/© 2015 Elsevier B.V. All rights reserved.
(M ¼ Fe, Co, Ni) nanoparticles [15], AuCo@MOF nanoparticles [16], PteCo@GO nanoparticles [17], NiPd@rGO nanoparticles [18], CuNi nanoparticles [19], and Cu@Co on rGO nanoparticles [20] are the examples of those type of catalysts. Very recently, poly(N-vinyl2-pyrrolidone (PVP))-protected bimetallic nanoparticles, such as rutheniumepalladium [21], platinumeruthenium [22], palladiumerhodium [23], and palladiumeplatinum [24] nanoparticles, have been shown to be highly efficient catalysts for hydrogen generation from boron compounds providing remarkable results. This study reports the first time use of PVP-protected rutheniumerhodium nanoparticles (RueRh@PVP NPs) as highly efficient catalysts for the hydrolysis of AB as a complementary study for the aforementioned catalysts. The catalysts were prepared by a modified alcohol reduction method [25], found to be stable as colloidal dispersions, and characterized by transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDX), ultravioletevisible spectroscopy (UVeVis), and X-ray photoelectron spectroscopy (XPS). Although the cost of noble metal catalysts is assumed to be high, the high catalytic activity of the RueRh@PVP nanoparticles makes them very promising candidates to be used as catalyst in developing efficient portable hydrogen generation systems using AB as solid hydrogen storage material since it would easily compensate the cost concerns.
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2. Experimental section 2.1. Materials Ruthenium(III) chloride trihydrate (RuCl.33H2O), rhodium(III) chloride trihydrate (RhCl.33H2O), poly(N-vinyl-2-pyrrolidone) (PVP-40), and ammonia borane (H3NBH3) were purchased from Aldrich. Ethanol was purchased from Merck. Deionized water was distilled by water purification system (Milli Q-pure WS). All glassware and teflon coated magnetic stir bars were rinsed with acetone, followed by copious washing with distilled water before drying in an oven at 150 C. 2.2. Preparation of RueRh@PVP nanoparticles RueRh@PVP nanoparticles were prepared by a modified alcohol reduction method in which PVP serves as both stabilizer and reducing agent. First, solutions of ruthenium (III) chloride trihydrate (0.25 mmol in 25 mL ethanol) and rhodium (III) chloride trihydrate (0.25 mmol in 25 mL water) were mixed and poly(Nvinyl-2-pyrrolidone) (PVP-40, 2.5 mmol of monomeric units) was added to this solution. Then, the mixed solution was refluxed at 363 K for 2 h RueRh@PVP nanoparticles are brownish black in color and stable at room temperature. The total concentration of both metals was kept as 5.0 mM in 50 mL of the mixed solution. 2.3. Characterization of RueRh@PVP nanoparticles 2.3.1. UVeVis analysis UVeVis spectra were recorded on a Cary 5000 (Varian) UVeVis spectrophotometer. A quartz cell with a part length of 1 cm was used and spectra were collected over the range of 200e900 nm. 2.3.2. TEM-EDX analysis Transmission Electron Microscopy (TEM) analysis was carried out using a JEOL-2010 microscope operating at 200 kV, fitted with a LaB6 filament and has lattice and theoretical point resolutions of 0.14 nm and 0.23 nm, respectively. Samples were examined at magnification between 100 and 400 K. One drop of dilute suspension of sample was deposited on the TEM grids and the solvent was then evaporated. The diameter of each particle was determined from the enlarged photographs. 2.3.3. X-ray photoelectron spectroscopy X-ray photoelectron spectrum (XPS) of the isolated nanoparticles was taken by using SPECS spectrometer equipped with a hemispherical analyzer and using monochromatic Mg-Ka radiation (1250 eV, the X-ray tube working at 15 kV and 350 W).
reaction flask to measure the volume of the hydrogen gas to be evolved from the reaction. In a typical experiment, 63.6 mg (2 mmol) of H3NBH3 was dissolved in 20 mL of water. The solution was transferred with a glass pipet into the reaction flask thermostated at 298 K. Then, aliquots of RueRh@PVP nanoparticles from the stock solution (5.0 mM) were added into the reaction flask. The experiment was started by closing the flask and the volume of hydrogen gas evolved was measured by recording the displacement of water level at the stirring speed of 1000 rpm. In addition to the volumetric measurement of the hydrogen evolution, the conversion of AB (d ¼ 23.9 ppm) [26] to metaborate (d ¼ 9 ppm) [27] was also checked by 11B NMR spectroscopy. 2.5. The effect of stirring speed on hydrogen generation rate The same experiments described in the Section 2.4 for the hydrogen generation from the hydrolysis of AB were performed at 298 K by varying the stirring speed (0, 200, 400, 600, 800, 1000, and 1200 rpm) to check how hydrogen generation rate from the hydrolysis of AB system was affected by stirring speed. The hydrogen generation rate was found to be independent of the stirring speed when it is higher than 800 rpm. This indicates that the system is in a non-mass transfer limitation regime since the present kinetic study was performed at the stirring speed of 1000 rpm. 2.6. The effect of PVP concentration on the catalytic activity of RueRh@PVP nanoparticles in the hydrolysis of AB In order to study the effect of PVP concentration on the catalytic activity of RueRh@PVP nanoparticles in the hydrolysis of AB (100 mM), hydrolysis reactions were carried out in the presence of catalysts prepared with different [PVP/Cat.] ratios (2, 6, 10, and 14). All the experiments were conducted in the same way described in Section 2.4. The optimum [PVP/Cat.] ratio was found to be 10. When this ratio is lower than 10, the catalytic activity of the catalyst is relatively low because PVP molecules do not cover the surface of nanoparticles effectively, leading to decreased catalytic activity by not preventing the agglomeration of nanoparticles. When it is higher than 10, catalytic activity of the catalyst starts to decrease since the surface of the nanoparticles may be wholly covered by PVP, blocking the active sites to be reached by substrate molecules. Therefore, optimum [PVP/Cat.] ratio was determined as 10 for further kinetic studies. 2.7. Determination of the activation energy of RueRh@PVP nanoparticles in the hydrolysis of AB
2.3.4. 11B NMR spectra 11 B NMR spectra were recorded on a Bruker Avance DPX 400 with an operating frequency of 128.15 MHz for 11B. At the end of the hydrolysis reaction, the resulting solutions were filtered and the filtrates used for taking 11B NMR spectra.
In a typical experiment, the hydrolysis of AB (100 mM) catalyzed by RueRh@PVP nanoparticles (0.3 mM) was performed by following the same procedure described in Section 2.4 at various temperatures (283, 288, 293, 298, and 303 K) to obtain the activation energy (Ea).
2.4. Catalytic evaluation of RueRh@PVP nanoparticles in the hydrolysis of AB
2.8. Durability of RueRh@PVP nanoparticles in the hydrolysis of AB
The catalytic activity of RueRh@PVP nanoparticles in the hydrolysis of AB in aqueous solution was determined by measuring the rate of hydrogen generation. In all experiments, a jacketed reaction flask (50 mL) containing a Teflon-coated stir bar was placed on a magnetic stirrer and thermostated to 298 K by circulating water through its jacket from a constant temperature bath. Then, a graduated glass tube filled with water was connected to the
The recyclability of RueRh@PVP nanoparticles in the hydrolysis of AB was determined by a series of experiments started with a 20 mL solution containing 0.3 mM RueRh@PVP nanoparticles and 100 mM AB at 298 K. When the complete conversion is achieved, another equivalent of AB was added to the reaction mixture immediately. The results were expressed as % initial catalytic activity of RueRh@PVP nanoparticles versus the number of catalytic runs in the hydrolysis of AB solution.
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3. Results and discussion 3.1. Preparation and characterization of RueRh@PVP nanoparticles
Fig. 1. UVeVis absorption spectra of the aqueous solutions of RuCl3$3H2O, RhCl3$3H2O, and RueRh@PVP nanoparticles.
RueRh@PVP nanoparticles were prepared from the coreduction of the mixture of ruthenium (III) chloride trihydrate and rhodium (III) chloride trihydrate by an alcohol reduction method in the presence of PVP in ethanol-water mixture at refluxing temperature. PVP serves as stabilizer and reducing agent. After refluxing for 2 h, the color of the solution turned to brownish black, indicating reduction of Ru3þ and Rh3þ ions to Ru0 and Rh0 to form bimetallic nanoparticles of them. Monitoring the UVeVis electronic absorption spectra of the solution provides the best way to follow this conversion. Fig. 1 shows the spectral change during the formation of RueRh nanoparticles from the reduction of corresponding ruthenium and rhodium salts by PVP. The absorption bands due to ded transitions in Ru3þ and Rh3þ ions completely disappear after refluxing the solution, indicating the complete reduction of corresponding ions.
Fig. 2. TEM images taken at different magnifications (a- 20 nm, b- 2 nm, d- 5 nm after 5th use in hydrolysis reaction) and EDX spectrum (c) of RueRh@PVP nanoparticles.
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The size, morphology and composition of RueRh@PVP nanoparticles were investigated by TEM-EDX analyses. Fig. 2 shows the TEM images taken at different magnifications (20 nm for Fig. 2a and 2 nm for Fig. 2b) and EDX spectrum (Fig. 2c) of RueRh@PVP nanoparticles, confirming 1:1 alloy structure which was also confirmed by ICP analysis. Alloy structure of the catalyst was further confirmed by measuring the lattice fringe length (Fig. 2b). The lattice fringe length of RueRh@PVP catalyst is 0.220 nm, which differs from both the Ru(101) crystal plane (0.206 nm) and the Rh(100) crystal plane (0.232 nm), showing the crystalline nature of RueRh alloy nanoparticles. The mean particle size was determined as 3.4 ± 2.0 nm from TEM image by counting 204 non-touching particles. The PVP-protected RueRh nanoparticles were isolated by evaporating all the solvent. The isolated samples of the catalyst were used for the XPS analysis of the catalyst surface. The main absorptions observed in the survey scan XPS (Fig. 3) belong to C 1s, Ru 3d, Ru 3p, Rh 3d, O 1s, and N 1s. The XPS spectrum for Rh 3d is characterized by a doublet containing a binding energy of 308.1 eV for 3d5/2 and 313.2 eV for 3d3/2, confirming the presence of Rh(0) [28]. Due to the strong overlap of the C1s and Ru 3d peaks around 285 eV, it is very difficult to analyze this region for ruthenium properly. The absorptions located at 462 eV and 485 eV for Ru 3p3/2 and Ru 3p1/2, respectively, are readily assigned to the Ru(0) [29]. There were some shifts to the higher binding energies due to the interaction between ruthenium and rhodium. There is no higher oxidation state peaks for both metals of the catalyst in the XPS spectra, indicating the protection of Ru(0) and Rh(0) species by the attachment of PVP during catalyst preparation procedure.
3.2. Catalytic evaluation of RueRh@PVP nanoparticles in the hydrolysis of AB RueRh@PVP nanoparticles were found to be highly efficient catalysts for the hydrolysis of AB. Fig. 4 shows the plots of mol (H2/ AB) versus time in the catalytic hydrolysis of 100 mM AB solutions in the presence of RueRh@PVP nanoparticles in different catalyst
Fig. 4. Plot of mol (H2/AB) versus time for the hydrolysis of 100 mM AB solutions in the presence of RueRh@PVP nanoparticles at different catalyst concentrations (0.1, 0.2, 0.3, 0.4, and 0.5 mM) at 298 K.
Fig. 3. Survey scan X-ray photoelectron spectrum of RueRh@PVP nanoparticles (Fig. 3a) with high resolution spectrum of Ru 3p (Fig. 3b) and (c) Rh 3d (Fig. 3c) regions.
M. Rakap / Journal of Alloys and Compounds 649 (2015) 1025e1030
concentrations (0.1, 0.2, 0.3, 0.4, and 0.5 mM) at 298 K. The linear hydrogen generation starts immediately without an induction period and continues until the complete hydrolysis of AB. Fig. 5 shows the plots of the volume of generated hydrogen gas versus time in the catalytic hydrolysis of 100 mM AB solutions in the presence of RueRh@PVP nanoparticles (0.3 mM) at various temperatures (283, 288, 293, 298, and 303 K). It is worth to note that using RueRh@PVP nanoparticles (0.3 mM) leads to complete hydrogen release (3.0 mol H2/mol AB) for the hydrolysis of AB within 210 s, corresponding to an average TOF value of 386 mol H2 (mol cat)1 min1 at 298 K. This average TOF value is among the best TOF values provided some bimetallic catalysts in the hydrolysis of AB (Table 1). Higher catalytic activity of RueRh@PVP nanoparticles stems from the synergistic effects of ruthenium and rhodium and the reduced particle size of the catalyst. The rate constants of hydrogen generation from the hydrolysis of AB were calculated from the linear portions of the plots given in Fig. 5 at five different temperatures and used for the calculation of the activation energy (Ea ¼ 47.4 ± 2.1 kJ/mol for the hydrolysis of AB) from the Arrhenius plot (Inset in Fig. 5) for hydrolysis reaction. This value of activation energy for the hydrolysis of AB is lower than
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the activation energies reported in the literature for the same reaction using many different catalysts: 52 kJ/mol for RuCu NPs [6], 51.6 kJ/mol for PteCo NPs [17], 51.3 kJ/mol for Cu@Co NPs [20], 54.5 kJ/mol for RuePd@PVP NPs [21], 56.3 kJ/mol for PteRu@PVP NPs [22], and 51.7 kJ/mol for PdePt@PVP NPs [24]; but still higher than 37.2 kJ/mol for NieRu NPs [5], 36.6 kJ/mol for Ru@Ni NPs [8], 38 kJ/mol for CuNi NPs [9], 45 kJ/mol for NiPd NPs [18], and 46.1 kJ/ mol for PdeRh@PVP NPs [23]. 3.3. The durability of RueRh@PVP nanoparticles in the hydrolysis of AB The durability of RueRh@PVP nanoparticles in the hydrolysis of AB was investigated by successive additions of AB after the first cycle of the hydrolysis reaction. The RueRh@PVP nanoparticles catalyst retains 68% of its initial catalytic activity in the hydrolysis of AB, even at the fifth run (Fig. 6). The decrease in the catalytic activity of RueRh@PVP nanoparticles in the hydrolysis of AB is due to the passivation of the surface of nanoparticles by increasing amount of metaborate, which decreases accessibility of active sites [30] and the aggregation of nanoparticles as shown in the TEM image of the catalyst taken after fifth run of the hydrolysis reaction (Fig. 2d, scale bar represents 5 nm). 4. Conclusions In summary, the study of the preparation, characterization and employment of RueRh@PVP nanoparticles as catalyst for the hydrolysis of AB has led to the following conclusions and insights:
Fig. 5. Plots of the volume of generated hydrogen gas versus time in the catalytic hydrolysis of 100 mM AB solutions in the presence of RueRh@PVP nanoparticles (0.3 mM) at various temperatures (283, 288, 293, 298, and 303 K). Inset shows the Arrhenius plot for the hydrolysis of AB (100 mM) catalyzed by 0.3 mM RueRh@PVP nanoparticles.
✓ RueRh@PVP nanoparticles can be easily prepared from the coreduction of corresponding ruthenium and rhodium salts by an alcohol reduction method. ✓ RueRh@PVP nanoparticles are highly efficient catalyst for hydrogen generation from the hydrolysis of AB providing high catalytic activity. ✓ They provide average TOF value of 386 mol H2 (mol cat)1 min1 and maximum hydrogen generation rate of 10,680 L H2 min1 (mol cat)1 for the hydrolysis of AB. ✓ Activation energy for the catalytic hydrolysis of AB in the presence of RueRh@PVP nanoparticles was calculated as 47.4 ± 2.1 kJ/mol.
Table 1 Activities in terms of TOF values of some bimetallic catalyst systems tested in hydrogen generation from the hydrolysis of AB. Catalyst
TOF (mol H2.mol catalyst1.min1)
Reference
Ni@Ru NPs RuCu NPs Ru@Ni NPs CueNi NPs Ru@Co NPs CoNi NPs CuCo NPs Ag/Pd NPs AuCo NPs PteCo NPs NiPd NPs CueNi NPs Cu@Co NPs RuePd@PVP NPs RuePt@PVP NPs PdeRh@PVP NPs PdePt@PVP NPs RueRh@PVP NPs
114.0 135.0 340.0 10.7 344.0 19.5 19.6 6.3 23.5 377.8 28.7 60.0 8.4 308.0 308.0 1333.0 125.0 386.0
[4] [7] [8] [9] [10] [11] [12] [14] [16] [17] [18] [19] [20] [21] [22] [23] [24] [This study]
Fig. 6. Retained % initial catalytic activity of 0.3 mM RueRh@PVP nanoparticles in the successive catalytic runs for the hydrolysis of 0.100 M AB at 298 K.
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✓ RueRh@PVP nanoparticles can be regarded as promising catalysts having high activity for practical applications to supply hydrogen from the hydrolysis of AB for proton exchange membrane fuel cells. Acknowledgments This study is supported by Research Fund of Yuzuncu Yil University (Project No: BAP-2013-FEN-B-014). References [1] J. Hannauer, O. Akdim, U.B. Demirci, C. Geantet, J.M. Herrmann, P. Miele, Q. Xu, Energy Environ. Sci. 4 (2011) 3355e3358. [2] Q. Xu, M. Chandra, J. Power Sources 163 (2006) 364e370. [3] T. Umegaki, J.M. Yan, X.B. Zhang, H. Shioyama, N. Kuriyama, Q. Xu, Int. J. Hydrogen Energy 34 (2009) 3816e3822. [4] G. Chen, S. Desinan, R. Nechache, R. Rosei, F. Rosei, D. Ma, Chem. Commun. 47 (2011) 6308e6310. [5] G. Chen, S. Desinan, R. Rosei, F. Rosei, D. Ma, Chem. Eur. J. 18 (2012) 7925e7930. [6] G.P. Rachiero, U.B. Demirci, P. Miele, Int. J. Hydrogen Energy 36 (2011) 7051e7065. [7] N. Cao, K. Hu, W. Luo, G. Cheng, J. Alloys Compd. 590 (2014) 241e246. [8] N. Cao, J. Su, W. Luo, G. Cheng, Int. J. Hydrogen Energy 39 (2014) 426e435. [9] Z.H. Lu, J. Li, G. Feng, Q. Yao, F. Zhang, R. Zhou, D. Tao, X. Chen, Z. Yu, Int. J. Hydrogen Energy 39 (2014) 13389e13395.
[10] N. Cao, J. Su, W. Luo, G. Cheng, Catal. Commun. 43 (2014) 47e51. [11] Y. Yang, F. Zhang, H. Wang, Q. Yao, X. Chen, Z.H. Lu, J. Nanomater. http:// dx.doi.org/10.1155/2014/294350. [12] J. Li, Q.L. Zhu, Q. Xu, Catal. Sci. Technol. 5 (2015) 525e530. [13] W. Jiao, X. Hu, H. Ren, P. Xu, R. Yu, J. Chen, X. Xing, J. Mater. Chem. A 2 (2014) 18171e18176. [14] Y. Tong, X. Lu, W. Sun, G. Nie, L. Yang, C. Wang, J. Power Sources 261 (2014) 221e226. [15] S. Wang, D. Zhang, Y. Ma, H. Zhang, J. Gao, Y. Nie, X. Sun, ACS Appl. Mater. Interfaces 6 (2014) 12429e12435. [16] J. Li, Q.L. Zhu, Q. Xu, Chem. Commun. 50 (2014) 5899e5901. [17] M. Li, J. Hu, Z. Chen, H. Lu, RSC Adv. 4 (2014) 41152e41158. € Metin, Int. J. Hydrogen Energy 39 (2014) 18863e18870. [18] N.S. Çiftci, O. [19] H. Yen, F. Kleitz, J. Mater. Chem. A 1 (2013) 14790e14796. [20] Y. Du, N. Cao, L. Yang, W. Luo, G. Cheng, New J. Chem. 37 (2013) 3035e3042. [21] M. Rakap, Int. J. Green Energy, http://dx.doi.org/10.1080/ 15435075.2014.895737. [22] M. Rakap, Appl. Catal. A Gen. 478 (2014) 15e20. [23] M. Rakap, Appl. Catal. B Env. 163 (2015) 129e134. [24] M. Rakap, J. Power Sources 276 (2015) 320e327. [25] N. Toshima, M. Harada, T. Yonezawa, K. Kushihashi, K. Asakura, J. Phys. Chem. 95 (1991) 7448e7453. [26] M. Chandra, Q. Xu, J. Power Sources 156 (2006) 190e194. [27] K.S. Eom, K.W. Cho, H.S. Kwon, Int. J. Hydrogen Energy 35 (2010) 181e186. [28] Y.V. Larichev, O.V. Netskina, O.V. Komova, V.I. Simagina, Int. J. Hydrogen Energy 35 (2010) 6501e6507. € [29] W. Wang, S. Guo, I. Lee, K. Ahmed, J. Zhong, Z. Favors, F. Zaera, M. Ozkan, C.S. € Ozkan, Sci. Rep., doi:10.1038/srep04452. [30] T.J. Clark, G.R. Whittell, I. Manners, Inorg. Chem. 46 (2007) 7522e7527.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
PVP-stabilized RueRh nanoparticles as highly efficient catalysts for hydrogen generation from hydrolysis of ammonia borane Murat Rakap Maritime Faculty, Yuzuncu Yil University, 65080, Van, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 May 2015 Received in revised form 23 July 2015 Accepted 27 July 2015 Available online 29 July 2015
Herein, the utilization of poly(N-vinyl-2-pyrrolidone)-protected rutheniumerhodium nanoparticles (3.4 ± 1.4 nm) as highly efficient catalysts in the hydrolysis of ammonia borane for hydrogen generation is reported. They are prepared by co-reduction of ruthenium and rhodium metal ions in ethanol/water mixture by an alcohol reduction method and characterized by transmission electron microscopy-energy dispersive X-ray spectroscopy, ultravioletevisible spectroscopy, and X-ray photoelectron spectroscopy. They are durable and highly efficient catalysts for hydrogen generation from the hydrolysis of ammonia borane even at very low concentrations and temperature, providing average turnover frequency of 386 mol H2 (mol cat)1 min1 and maximum hydrogen generation rate of 10,680 L H2 min1 (mol cat)1. Poly(N-vinyl-2-pyrrolidone)-protected rutheniumerhodium nanoparticles also provide activation energy of 47.4 ± 2.1 kJ/mol for the hydrolysis of ammonia borane. © 2015 Elsevier B.V. All rights reserved.
Keywords: Ruthenium Rhodium Nanoparticle Ammonia borane Hydrogen
1. Introduction Employment of chemical hydrides as efficient and safe hydrogen storage materials is one of the most promising approaches [1]. For this purpose, ammonia borane (H3NBH3, AB) has recently been employed as a solid hydrogen storage material due to its high hydrogen content (19.6 wt %), high solubility and stability in water at room temperature [2,3]. AB is able to release hydrogen upon hydrolysis at room temperature in the presence of suitable catalysts according to Eq. (1): catalyst
H3 NBH3 ðaqÞ þ 2H2 OðlÞ !NHþ 4 ðaqÞ þ BO2 ðaqÞ þ 3H2 ðgÞ
(1) Recently, different kinds of bimetallic nanoparticle-type catalysts have been tested for hydrogen generation from the hydrolysis of AB since the addition of second element to the monometallic nanoparticles definitely improves the catalytic properties. Ni@Ru coreeshell nanoparticles [4], NieRu alloy nanoparticles [5], RuCo and RuCu on ɤ-Al2O3 [6], RuCu on graphene [7], Ru@Ni coreeshell nanoparticles [8], CueNi on MCM-41 [9], Ru@Co on graphene [10], CoNi@rGO [11], CuCo@MOF nanoparticles [12], Ni/Pt hollow nanospheres [13], Ag/Pd@nanofiber nanoparticles [14], Pt-M
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.jallcom.2015.07.249 0925-8388/© 2015 Elsevier B.V. All rights reserved.
(M ¼ Fe, Co, Ni) nanoparticles [15], AuCo@MOF nanoparticles [16], PteCo@GO nanoparticles [17], NiPd@rGO nanoparticles [18], CuNi nanoparticles [19], and Cu@Co on rGO nanoparticles [20] are the examples of those type of catalysts. Very recently, poly(N-vinyl2-pyrrolidone (PVP))-protected bimetallic nanoparticles, such as rutheniumepalladium [21], platinumeruthenium [22], palladiumerhodium [23], and palladiumeplatinum [24] nanoparticles, have been shown to be highly efficient catalysts for hydrogen generation from boron compounds providing remarkable results. This study reports the first time use of PVP-protected rutheniumerhodium nanoparticles (RueRh@PVP NPs) as highly efficient catalysts for the hydrolysis of AB as a complementary study for the aforementioned catalysts. The catalysts were prepared by a modified alcohol reduction method [25], found to be stable as colloidal dispersions, and characterized by transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDX), ultravioletevisible spectroscopy (UVeVis), and X-ray photoelectron spectroscopy (XPS). Although the cost of noble metal catalysts is assumed to be high, the high catalytic activity of the RueRh@PVP nanoparticles makes them very promising candidates to be used as catalyst in developing efficient portable hydrogen generation systems using AB as solid hydrogen storage material since it would easily compensate the cost concerns.
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2. Experimental section 2.1. Materials Ruthenium(III) chloride trihydrate (RuCl.33H2O), rhodium(III) chloride trihydrate (RhCl.33H2O), poly(N-vinyl-2-pyrrolidone) (PVP-40), and ammonia borane (H3NBH3) were purchased from Aldrich. Ethanol was purchased from Merck. Deionized water was distilled by water purification system (Milli Q-pure WS). All glassware and teflon coated magnetic stir bars were rinsed with acetone, followed by copious washing with distilled water before drying in an oven at 150 C. 2.2. Preparation of RueRh@PVP nanoparticles RueRh@PVP nanoparticles were prepared by a modified alcohol reduction method in which PVP serves as both stabilizer and reducing agent. First, solutions of ruthenium (III) chloride trihydrate (0.25 mmol in 25 mL ethanol) and rhodium (III) chloride trihydrate (0.25 mmol in 25 mL water) were mixed and poly(Nvinyl-2-pyrrolidone) (PVP-40, 2.5 mmol of monomeric units) was added to this solution. Then, the mixed solution was refluxed at 363 K for 2 h RueRh@PVP nanoparticles are brownish black in color and stable at room temperature. The total concentration of both metals was kept as 5.0 mM in 50 mL of the mixed solution. 2.3. Characterization of RueRh@PVP nanoparticles 2.3.1. UVeVis analysis UVeVis spectra were recorded on a Cary 5000 (Varian) UVeVis spectrophotometer. A quartz cell with a part length of 1 cm was used and spectra were collected over the range of 200e900 nm. 2.3.2. TEM-EDX analysis Transmission Electron Microscopy (TEM) analysis was carried out using a JEOL-2010 microscope operating at 200 kV, fitted with a LaB6 filament and has lattice and theoretical point resolutions of 0.14 nm and 0.23 nm, respectively. Samples were examined at magnification between 100 and 400 K. One drop of dilute suspension of sample was deposited on the TEM grids and the solvent was then evaporated. The diameter of each particle was determined from the enlarged photographs. 2.3.3. X-ray photoelectron spectroscopy X-ray photoelectron spectrum (XPS) of the isolated nanoparticles was taken by using SPECS spectrometer equipped with a hemispherical analyzer and using monochromatic Mg-Ka radiation (1250 eV, the X-ray tube working at 15 kV and 350 W).
reaction flask to measure the volume of the hydrogen gas to be evolved from the reaction. In a typical experiment, 63.6 mg (2 mmol) of H3NBH3 was dissolved in 20 mL of water. The solution was transferred with a glass pipet into the reaction flask thermostated at 298 K. Then, aliquots of RueRh@PVP nanoparticles from the stock solution (5.0 mM) were added into the reaction flask. The experiment was started by closing the flask and the volume of hydrogen gas evolved was measured by recording the displacement of water level at the stirring speed of 1000 rpm. In addition to the volumetric measurement of the hydrogen evolution, the conversion of AB (d ¼ 23.9 ppm) [26] to metaborate (d ¼ 9 ppm) [27] was also checked by 11B NMR spectroscopy. 2.5. The effect of stirring speed on hydrogen generation rate The same experiments described in the Section 2.4 for the hydrogen generation from the hydrolysis of AB were performed at 298 K by varying the stirring speed (0, 200, 400, 600, 800, 1000, and 1200 rpm) to check how hydrogen generation rate from the hydrolysis of AB system was affected by stirring speed. The hydrogen generation rate was found to be independent of the stirring speed when it is higher than 800 rpm. This indicates that the system is in a non-mass transfer limitation regime since the present kinetic study was performed at the stirring speed of 1000 rpm. 2.6. The effect of PVP concentration on the catalytic activity of RueRh@PVP nanoparticles in the hydrolysis of AB In order to study the effect of PVP concentration on the catalytic activity of RueRh@PVP nanoparticles in the hydrolysis of AB (100 mM), hydrolysis reactions were carried out in the presence of catalysts prepared with different [PVP/Cat.] ratios (2, 6, 10, and 14). All the experiments were conducted in the same way described in Section 2.4. The optimum [PVP/Cat.] ratio was found to be 10. When this ratio is lower than 10, the catalytic activity of the catalyst is relatively low because PVP molecules do not cover the surface of nanoparticles effectively, leading to decreased catalytic activity by not preventing the agglomeration of nanoparticles. When it is higher than 10, catalytic activity of the catalyst starts to decrease since the surface of the nanoparticles may be wholly covered by PVP, blocking the active sites to be reached by substrate molecules. Therefore, optimum [PVP/Cat.] ratio was determined as 10 for further kinetic studies. 2.7. Determination of the activation energy of RueRh@PVP nanoparticles in the hydrolysis of AB
2.3.4. 11B NMR spectra 11 B NMR spectra were recorded on a Bruker Avance DPX 400 with an operating frequency of 128.15 MHz for 11B. At the end of the hydrolysis reaction, the resulting solutions were filtered and the filtrates used for taking 11B NMR spectra.
In a typical experiment, the hydrolysis of AB (100 mM) catalyzed by RueRh@PVP nanoparticles (0.3 mM) was performed by following the same procedure described in Section 2.4 at various temperatures (283, 288, 293, 298, and 303 K) to obtain the activation energy (Ea).
2.4. Catalytic evaluation of RueRh@PVP nanoparticles in the hydrolysis of AB
2.8. Durability of RueRh@PVP nanoparticles in the hydrolysis of AB
The catalytic activity of RueRh@PVP nanoparticles in the hydrolysis of AB in aqueous solution was determined by measuring the rate of hydrogen generation. In all experiments, a jacketed reaction flask (50 mL) containing a Teflon-coated stir bar was placed on a magnetic stirrer and thermostated to 298 K by circulating water through its jacket from a constant temperature bath. Then, a graduated glass tube filled with water was connected to the
The recyclability of RueRh@PVP nanoparticles in the hydrolysis of AB was determined by a series of experiments started with a 20 mL solution containing 0.3 mM RueRh@PVP nanoparticles and 100 mM AB at 298 K. When the complete conversion is achieved, another equivalent of AB was added to the reaction mixture immediately. The results were expressed as % initial catalytic activity of RueRh@PVP nanoparticles versus the number of catalytic runs in the hydrolysis of AB solution.
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3. Results and discussion 3.1. Preparation and characterization of RueRh@PVP nanoparticles
Fig. 1. UVeVis absorption spectra of the aqueous solutions of RuCl3$3H2O, RhCl3$3H2O, and RueRh@PVP nanoparticles.
RueRh@PVP nanoparticles were prepared from the coreduction of the mixture of ruthenium (III) chloride trihydrate and rhodium (III) chloride trihydrate by an alcohol reduction method in the presence of PVP in ethanol-water mixture at refluxing temperature. PVP serves as stabilizer and reducing agent. After refluxing for 2 h, the color of the solution turned to brownish black, indicating reduction of Ru3þ and Rh3þ ions to Ru0 and Rh0 to form bimetallic nanoparticles of them. Monitoring the UVeVis electronic absorption spectra of the solution provides the best way to follow this conversion. Fig. 1 shows the spectral change during the formation of RueRh nanoparticles from the reduction of corresponding ruthenium and rhodium salts by PVP. The absorption bands due to ded transitions in Ru3þ and Rh3þ ions completely disappear after refluxing the solution, indicating the complete reduction of corresponding ions.
Fig. 2. TEM images taken at different magnifications (a- 20 nm, b- 2 nm, d- 5 nm after 5th use in hydrolysis reaction) and EDX spectrum (c) of RueRh@PVP nanoparticles.
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The size, morphology and composition of RueRh@PVP nanoparticles were investigated by TEM-EDX analyses. Fig. 2 shows the TEM images taken at different magnifications (20 nm for Fig. 2a and 2 nm for Fig. 2b) and EDX spectrum (Fig. 2c) of RueRh@PVP nanoparticles, confirming 1:1 alloy structure which was also confirmed by ICP analysis. Alloy structure of the catalyst was further confirmed by measuring the lattice fringe length (Fig. 2b). The lattice fringe length of RueRh@PVP catalyst is 0.220 nm, which differs from both the Ru(101) crystal plane (0.206 nm) and the Rh(100) crystal plane (0.232 nm), showing the crystalline nature of RueRh alloy nanoparticles. The mean particle size was determined as 3.4 ± 2.0 nm from TEM image by counting 204 non-touching particles. The PVP-protected RueRh nanoparticles were isolated by evaporating all the solvent. The isolated samples of the catalyst were used for the XPS analysis of the catalyst surface. The main absorptions observed in the survey scan XPS (Fig. 3) belong to C 1s, Ru 3d, Ru 3p, Rh 3d, O 1s, and N 1s. The XPS spectrum for Rh 3d is characterized by a doublet containing a binding energy of 308.1 eV for 3d5/2 and 313.2 eV for 3d3/2, confirming the presence of Rh(0) [28]. Due to the strong overlap of the C1s and Ru 3d peaks around 285 eV, it is very difficult to analyze this region for ruthenium properly. The absorptions located at 462 eV and 485 eV for Ru 3p3/2 and Ru 3p1/2, respectively, are readily assigned to the Ru(0) [29]. There were some shifts to the higher binding energies due to the interaction between ruthenium and rhodium. There is no higher oxidation state peaks for both metals of the catalyst in the XPS spectra, indicating the protection of Ru(0) and Rh(0) species by the attachment of PVP during catalyst preparation procedure.
3.2. Catalytic evaluation of RueRh@PVP nanoparticles in the hydrolysis of AB RueRh@PVP nanoparticles were found to be highly efficient catalysts for the hydrolysis of AB. Fig. 4 shows the plots of mol (H2/ AB) versus time in the catalytic hydrolysis of 100 mM AB solutions in the presence of RueRh@PVP nanoparticles in different catalyst
Fig. 4. Plot of mol (H2/AB) versus time for the hydrolysis of 100 mM AB solutions in the presence of RueRh@PVP nanoparticles at different catalyst concentrations (0.1, 0.2, 0.3, 0.4, and 0.5 mM) at 298 K.
Fig. 3. Survey scan X-ray photoelectron spectrum of RueRh@PVP nanoparticles (Fig. 3a) with high resolution spectrum of Ru 3p (Fig. 3b) and (c) Rh 3d (Fig. 3c) regions.
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concentrations (0.1, 0.2, 0.3, 0.4, and 0.5 mM) at 298 K. The linear hydrogen generation starts immediately without an induction period and continues until the complete hydrolysis of AB. Fig. 5 shows the plots of the volume of generated hydrogen gas versus time in the catalytic hydrolysis of 100 mM AB solutions in the presence of RueRh@PVP nanoparticles (0.3 mM) at various temperatures (283, 288, 293, 298, and 303 K). It is worth to note that using RueRh@PVP nanoparticles (0.3 mM) leads to complete hydrogen release (3.0 mol H2/mol AB) for the hydrolysis of AB within 210 s, corresponding to an average TOF value of 386 mol H2 (mol cat)1 min1 at 298 K. This average TOF value is among the best TOF values provided some bimetallic catalysts in the hydrolysis of AB (Table 1). Higher catalytic activity of RueRh@PVP nanoparticles stems from the synergistic effects of ruthenium and rhodium and the reduced particle size of the catalyst. The rate constants of hydrogen generation from the hydrolysis of AB were calculated from the linear portions of the plots given in Fig. 5 at five different temperatures and used for the calculation of the activation energy (Ea ¼ 47.4 ± 2.1 kJ/mol for the hydrolysis of AB) from the Arrhenius plot (Inset in Fig. 5) for hydrolysis reaction. This value of activation energy for the hydrolysis of AB is lower than
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the activation energies reported in the literature for the same reaction using many different catalysts: 52 kJ/mol for RuCu NPs [6], 51.6 kJ/mol for PteCo NPs [17], 51.3 kJ/mol for Cu@Co NPs [20], 54.5 kJ/mol for RuePd@PVP NPs [21], 56.3 kJ/mol for PteRu@PVP NPs [22], and 51.7 kJ/mol for PdePt@PVP NPs [24]; but still higher than 37.2 kJ/mol for NieRu NPs [5], 36.6 kJ/mol for Ru@Ni NPs [8], 38 kJ/mol for CuNi NPs [9], 45 kJ/mol for NiPd NPs [18], and 46.1 kJ/ mol for PdeRh@PVP NPs [23]. 3.3. The durability of RueRh@PVP nanoparticles in the hydrolysis of AB The durability of RueRh@PVP nanoparticles in the hydrolysis of AB was investigated by successive additions of AB after the first cycle of the hydrolysis reaction. The RueRh@PVP nanoparticles catalyst retains 68% of its initial catalytic activity in the hydrolysis of AB, even at the fifth run (Fig. 6). The decrease in the catalytic activity of RueRh@PVP nanoparticles in the hydrolysis of AB is due to the passivation of the surface of nanoparticles by increasing amount of metaborate, which decreases accessibility of active sites [30] and the aggregation of nanoparticles as shown in the TEM image of the catalyst taken after fifth run of the hydrolysis reaction (Fig. 2d, scale bar represents 5 nm). 4. Conclusions In summary, the study of the preparation, characterization and employment of RueRh@PVP nanoparticles as catalyst for the hydrolysis of AB has led to the following conclusions and insights:
Fig. 5. Plots of the volume of generated hydrogen gas versus time in the catalytic hydrolysis of 100 mM AB solutions in the presence of RueRh@PVP nanoparticles (0.3 mM) at various temperatures (283, 288, 293, 298, and 303 K). Inset shows the Arrhenius plot for the hydrolysis of AB (100 mM) catalyzed by 0.3 mM RueRh@PVP nanoparticles.
✓ RueRh@PVP nanoparticles can be easily prepared from the coreduction of corresponding ruthenium and rhodium salts by an alcohol reduction method. ✓ RueRh@PVP nanoparticles are highly efficient catalyst for hydrogen generation from the hydrolysis of AB providing high catalytic activity. ✓ They provide average TOF value of 386 mol H2 (mol cat)1 min1 and maximum hydrogen generation rate of 10,680 L H2 min1 (mol cat)1 for the hydrolysis of AB. ✓ Activation energy for the catalytic hydrolysis of AB in the presence of RueRh@PVP nanoparticles was calculated as 47.4 ± 2.1 kJ/mol.
Table 1 Activities in terms of TOF values of some bimetallic catalyst systems tested in hydrogen generation from the hydrolysis of AB. Catalyst
TOF (mol H2.mol catalyst1.min1)
Reference
Ni@Ru NPs RuCu NPs Ru@Ni NPs CueNi NPs Ru@Co NPs CoNi NPs CuCo NPs Ag/Pd NPs AuCo NPs PteCo NPs NiPd NPs CueNi NPs Cu@Co NPs RuePd@PVP NPs RuePt@PVP NPs PdeRh@PVP NPs PdePt@PVP NPs RueRh@PVP NPs
114.0 135.0 340.0 10.7 344.0 19.5 19.6 6.3 23.5 377.8 28.7 60.0 8.4 308.0 308.0 1333.0 125.0 386.0
[4] [7] [8] [9] [10] [11] [12] [14] [16] [17] [18] [19] [20] [21] [22] [23] [24] [This study]
Fig. 6. Retained % initial catalytic activity of 0.3 mM RueRh@PVP nanoparticles in the successive catalytic runs for the hydrolysis of 0.100 M AB at 298 K.
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✓ RueRh@PVP nanoparticles can be regarded as promising catalysts having high activity for practical applications to supply hydrogen from the hydrolysis of AB for proton exchange membrane fuel cells. Acknowledgments This study is supported by Research Fund of Yuzuncu Yil University (Project No: BAP-2013-FEN-B-014). References [1] J. Hannauer, O. Akdim, U.B. Demirci, C. Geantet, J.M. Herrmann, P. Miele, Q. Xu, Energy Environ. Sci. 4 (2011) 3355e3358. [2] Q. Xu, M. Chandra, J. Power Sources 163 (2006) 364e370. [3] T. Umegaki, J.M. Yan, X.B. Zhang, H. Shioyama, N. Kuriyama, Q. Xu, Int. J. Hydrogen Energy 34 (2009) 3816e3822. [4] G. Chen, S. Desinan, R. Nechache, R. Rosei, F. Rosei, D. Ma, Chem. Commun. 47 (2011) 6308e6310. [5] G. Chen, S. Desinan, R. Rosei, F. Rosei, D. Ma, Chem. Eur. J. 18 (2012) 7925e7930. [6] G.P. Rachiero, U.B. Demirci, P. Miele, Int. J. Hydrogen Energy 36 (2011) 7051e7065. [7] N. Cao, K. Hu, W. Luo, G. Cheng, J. Alloys Compd. 590 (2014) 241e246. [8] N. Cao, J. Su, W. Luo, G. Cheng, Int. J. Hydrogen Energy 39 (2014) 426e435. [9] Z.H. Lu, J. Li, G. Feng, Q. Yao, F. Zhang, R. Zhou, D. Tao, X. Chen, Z. Yu, Int. J. Hydrogen Energy 39 (2014) 13389e13395.
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