- Email: [email protected]
Core–shell nanospheres Pt@SiO2 for catalytic hydrogen production
Applied Surface Science 341 (2015) 185–189
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Core–shell nanospheres Pt@SiO2 for catalytic hydrogen production Yujuan Hu, Yuqing Wang, Zhang-Hui Lu ∗ , Xiangshu Chen ∗∗ , Lihua Xiong Jiangxi Inorganic Membrane Materials Engineering Research Centre, Key Laboratory of Functional Small Organic Molecule (Ministry of Education), College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, China
a r t i c l e
i n f o
Article history: Received 17 November 2014 Received in revised form 13 February 2015 Accepted 14 February 2015 Available online 4 March 2015 Keywords: Core–shell nanoparticles Hydrolysis Ammonia borane Hydrogen
a b s t r a c t Ultrafine platinum nanoparticles (NPs) embedded in silica nanospheres (Pt@SiO2 ) have been synthesized in a NP-5/cyclohexane reversed-micelle system followed by NaBH4 reduction. The as-synthesized core–shell nanocatalysts Pt@SiO2 were characterized by scanning electron microscopy, transmission electron microscopes, X-ray powder diffraction analysis, energy dispersive X-ray spectrometer and nitrogen adsorption–desorption investigations. Interestingly, the as-synthesized core–shell nanocatalysts Pt@SiO2 showed an excellent catalytic performance in hydrogen generation from the hydrolysis of ammonia borane (BH3 NH3 , AB) at room temperature. Especially, the catalytic performance of the Pt@SiO2 remained almost unchanged after the five recycles and even after the heat treatment (673 K), because the silica shells inhibit aggregation or deformation of the metal cores. Besides, the kinetic studies showed that the catalytic hydrolysis of AB was first order with respect to the catalyst concentration and zero order with respect to the substrate concentration, respectively. The excellent catalytic activity and stability of Pt@SiO2 can make it have a bright future in the practical application. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, hydrogen is one of the most attractive energy carriers for a long-term solution to current energy problems owing to its high calorific value and non-polluting nature. The safe and efficient storage of hydrogen is of great significance to fulfill the goal of hydrogen-powered transport systems. In recent years, ammonia borane (NH3 BH3 , AB) has drawn much attention as a good candidate for hydrogen reservoir due to its high hydrogen capacity of 19.6 wt%, good stability and non-toxicity [1–4]. Catalytic hydrolysis of AB in the presence of a suitable catalyst can release three mol of hydrogen per mol of AB at ambient temperature, which presents a high hydrogen capacity up to 9.0 wt% of the starting materials (AB and H2 O), thus making itself an effective approach for the release of hydrogen stored in AB [5–11]. On this count, it is the key point to select a suitable catalyst. A number of catalysts have been tested in hydrogen generation from the hydrolysis of AB, including transition metal NPs such as platinum, rhodium, ruthenium, palladium, cobalt and nickel NPs [2,3]. Although rhodium, platinum and ruthenium metal NPs have high price and limited abundance, they are superior to the non-noble metal NPs due to their long lifetime and high
activity in the hydrolysis of AB. Therefore the use of noble metal NPs as catalysts has been intensively studied in recent years [12–24]. However, big challenges still remain. To make the AB complex a practical hydrogen reservoir for “on board” applications, highly efficient and stable catalysts are highly desired. Platinum-based catalysts have been attracted much attention in the past decades as the excellent and versatile catalysts in various important reactions [16–23,25,26]. Taking into account the practical applications of this system, various solid supports, such as silica, carbon, metal oxide and zeolite, have been employed to achieve high Pt utilization through preparing highly dispersed Pt NPs. However, the aggregation of metal particles accompanied by the loss of catalytic activity is still a great challenge. The encapsulation of metal particles into the supports with core–shell-type structures has attracted great attention owing to its high stability [14,15,27–32]. Herein, we report a one-pot synthesis of core–shell nanospheres Pt@SiO2 , which show high catalytic performance and excellent recycle stability for the hydrogen generation from AB hydrolysis at room temperature.
2. Experimental 2.1. Chemicals
∗ Corresponding author. Tel.: +86 791 88121974; fax: +86 791 88121974. ∗∗ Corresponding author. E-mail addresses: [email protected] (Z.-H. Lu), [email protected] (X. Chen). http://dx.doi.org/10.1016/j.apsusc.2015.02.094 0169-4332/© 2015 Elsevier B.V. All rights reserved.
Potassium tetrachloroplatinate (II) (K2 PtCl4 , J&K, 99.95%), tetraethoxysilane (TEOS, Sigma–Aldrich, 98%), polyethylene
186
Y. Hu et al. / Applied Surface Science 341 (2015) 185–189
glycolmono-4-nonylphenyl ether n ≈ 5 (NP-5, Tokyo Chemical Industry Co., Ltd), ammonia solution (NH3 ·H2 O, Nanchang Chemical Works, 28%), cyclohexane (C6 H12 , Tianjin Fuchen Chemical Reagent, >99.5%), methanol (CH3 OH, Tianjin Fuchen Chemical Reagent, 99.5%), and acetone ((CH3 )2 CO, Nanchang Chemical Works, >99.5%) were used as received. Ammonia borane (NH3 BH3 , 90%) and sodium borohydride (NaBH4 , 99%) were purchased from Aldrich. All chemicals were used without any purification. Ultrapure deionized water with the specific resistance of 18.3 M cm was generated by reversed osmosis followed by ion exchange and filtration. 2.2. Synthesis of Pt@SiO2 The reverse micelle system used in this study composed of water, surfactant and organic solvent. 2.16 mL of aqueous K2 PtCl4 solution (containing 74.75 mg K2 PtCl4 , corresponding to 5 wt% Pt loading) was added into 480 mL of NP-5 (20.16 g)/cyclohexane solution, and the mixture was stirred at room temperature for 15 h. Next, 2.16 mL of ammonium hydroxide solution was injected rapidly into the mixture under continuously stirring and after 2 h TEOS (2.49 mL) was added. After two days of stirring at room temperature, the as-synthesized silica nanospheres were isolated by adding methanol followed by washing with cyclohexane and acetone to remove surfactant and unreacted chemicals, and centrifugation at 10,000 rpm for 8 min. Then, the as-synthesized Pt@SiO2 NPs were reduced by 20 mL of freshly prepared solution of NaBH4 (100 mg). Finally, the solid Pt@SiO2 NPs were received by drying the samples in vacuum oven at 40 ◦ C for overnight. In order to test the thermally stable of Pt@SiO2 , the as-synthesized NPs were calcined at 400 ◦ C for 2 h. 2.3. Synthesis of Pt/SiO2 The Pt/SiO2 NPs in this study were prepared by the conventional impregnation. The Pt content was kept to be about 5 wt%. 1.01 mg K2 PtCl4 and 19 mg SiO2 were dissolved in 10 mL of distilled water for 2 h. Then, the sample was reduced by 5 mg NaBH4 . Finally, the received Pt/SiO2 NPs were used to catalytic hydrolysis of AB (34.29 mg). 2.4. Synthesis of Pt NPs 1.01 mg K2 PtCl4 was dissolved in 10 mL of distilled water and 5 mg NaBH4 was added into the solution. Then, the received Pt NPs were used to catalytic hydrolysis of AB (34.29 mg).
2.6. Catalytic hydrolysis of AB by Pt@SiO2 (C 400) NPs The catalytic activity of Pt@SiO2 (C 400) NPs toward AB hydrolysis was determined by measuring the rate of hydrogen generation in a typical experiment described in Section 2.5. The catalytic performance of Pt@SiO2 has no significant changed after the heat treatment with 400 ◦ C. 2.7. Kinetic study of Pt@SiO2 in hydrolysis of AB In order to establish the rate law for the hydrolytic dehydrogenation of AB catalyzed by Pt@SiO2 , two different sets of experiment were performed in the same way as described in Section 2.5. In the first set of experiments, keeping the initial AB concentration constant at 100 mM, the amount of Pt@SiO2 catalyst (5 wt%) was varied in 0.082 mM, 0.163 mM, 0.245 mM and 0.326 mM at 25 ◦ C. In the second set of experiments, keeping the initial Pt@SiO2 concentration (5 wt%) constant at 0.245 mM, the amount of AB was varied in 100 mM, 150 mM, 200 mM and 300 mM at 25 ◦ C. 2.8. Recycle test of Pt@SiO2 in hydrolysis of AB The reactions were repeated five times under ambient atmosphere at room temperature. After hydrolysis reaction test, the catalysts were separated from the reaction solution by centrifugation, washed with water for three times and dried in vacuum oven at 40 ◦ C overnight. 2.9. Characterization The size and morphology composition of the samples were characterized by field emission scanning electron microscopy (FE-SEM, Hitachi, SU8020) at acceleration voltages of 3–10 kV, field emission transmission electron microscope (FE-TEM, FEI, Tecnai G2 F20 (200 kV)) and an energy dispersive X-ray spectrometer (EDS) attached on FE-TEM for elemental analysis. The TEM samples were prepared by depositing one or two droplets of the nanoparticle suspensions on to the amorphous carbon coated copper grids. The structures were characterized by Powder X-ray diffraction (XRD, Rigaku RINT-2200 X-ray diffractometer with a CuKa source (40 kV, 20 mA)) and X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI, USA) on a K-alpha spectrometer with monochromatic Al K␣ radiation (hv = 1486.6 eV). Brunauer–Emmett–Teller (BET, BEL Japan, Inc., BELSORP-mini II) surface area and pore parameters analysis were characterized by N2 adsorption–desorption. 3. Results and discussion 3.1. Characterization of the samples
2.5. Catalytic hydrolysis of AB by as-prepared Pt@SiO2 In a typical experiment, the catalytic activity of Pt@SiO2 NPs toward AB hydrolysis was determined by measuring the rate of hydrogen generation. 9.546 mg of Pt@SiO2 NPs and 10 mL of deionized water were added into a two-necked round-bottomed flask, which was kept in a water bath heating at 25 ◦ C. A gas buret was connected to one neck of the flask to measure the volume of hydrogen, and 34.29 mg of AB was introduced into the reaction flask through the other neck. The reaction was started when AB was added into the flask under electric stirring at room temperature. The evolution of hydrogen was monitored using the gas buret. The hydrolysis of NH3 BH3 can be briefly expressed as follows: NH3 BH3 + 2H2 O → NH4 + + BO2 − + 3H2 . The volume of released hydrogen gas was monitored by recording time for the displacement of water. The reaction was completed when there was no more hydrogen gas generated.
The as-synthesized NPs were characterized by SEM and TEM. As shown in Fig. 1a, SEM image shows that the size of the assynthesized Pt@SiO2 is quite uniform with an average diameter of ∼25 nm. The monodisperse spherical morphology of Pt@SiO2 can be further confirmed by the TEM image (Fig. 1b). It can be seen from Fig. 1b that the single Pt NP as a core with a diameter of ∼4 nm is effectively embedded into the silica nanosphere (∼25 nm). However, the large agglomerate/aggregate Pt NPs were observed in Pt/SiO2 (Pt with an average diameter of ∼16 nm) and Pt NPs in Fig. S1 (supplementary data), which may lead to a decrease in the catalytic activity and reusability. The high-resolution TEM (HRTEM) image (Fig. 1c) of Pt@SiO2 demonstrates the polycrystalline structure of the Pt core with fringes associated with a d-spacing of ∼0.22 nm, corresponding to the (1 1 1) plane of the Pt face-centercubic (fcc) structure. The EDS of Pt@SiO2 shows that the sample includes elements Pt, Si, O and Cu which is from TEM grids in
Y. Hu et al. / Applied Surface Science 341 (2015) 185–189
187
Fig. 1. (a) SEM, (b) TEM, (c) HRTEM images of Pt@SiO2 and (d) TEM of Pt@SiO2 after five recycles.
Fig. 2. (a) EDS and (b) XRD of Pt@SiO2 .
Fig. 2a. The X-ray diffraction (XRD) patterns of the samples before and after hydrolysis of AB are given in Fig. 2b. The strong and broad diffraction peak at 2 = 15–35◦ observed in the XRD patterns is attributed to the amorphous silica. And other diffraction peaks in XRD patterns could be indexed to those of Pt fcc structure (JCPDS No. 01-1194). A nitrogen adsorption–desorption study (Fig. 3 and Fig. S3) shows that the Pt@SiO2 , Pt/SiO2 and Pt NPs have a Brunauer–Emmett–Teller (BET) surface area of 104, 118 and 3 m2 g−1 , respectively. The isotherms of Pt@SiO2 NPs exhibit a type IV isotherm with a hysteresis loop at a P/P0 region of 0.8–1.0 (Fig. 3), indicating the existence of mesopores in the sample, confirmed by the pore size (0.48 cm3 g−1 ) distribution of core–shell sample (Fig. S2). The XPS pattern shows the significant Pt 4f signal corresponding to the binding energy of Pt in Fig. S4. The Pt 4f7/2 and Pt 4f5/2 binding energies correspond to Pt (0).
in Fig. 4a. The evolution of hydrogen is finished in 7.72, 55.98 and 111 min, respectively, in the presence of the as-synthesized Pt@SiO2 , Pt/SiO2 and Pt NPs at room temperature. The hydrogen generation rates from AB are in the order of Pt@SiO2 > Pt/SiO2 > Pt
3.2. Catalytic properties Catalytic activities of Pt@SiO2 NPs together with Pt, SiO2 and Pt/SiO2 (Pt on Silica) have been studied for the hydrolytic dehydrogenation of AB at room temperature, and the results are show
Fig. 3. The nitrogen adsorption-desorption isotherms of Pt@SiO2 .
188
Y. Hu et al. / Applied Surface Science 341 (2015) 185–189
Fig. 4. (a) Plots of the volume of hydrogen generation from AB (100 mM, 10 mL) hydrolysis as a function of time catalyzed by Pt@SiO2 , Pt/SiO2 , Pt and SiO2 at 25 ◦ C, respectively. (b) Recyclability of Pt@SiO2 .
with the same Pt content while the particle sizes of these catalysts are in the order of Pt@SiO2 < Pt/SiO2 < Pt (Fig. 1 and Fig. S1). Among all of the catalysts, Pt@SiO2 presents the highest catalytic activity to release a stoichiometric amount of hydrogen from aqueous AB (H2 /AB = 3.0) with a turnover frequency (TOF) value of 158.6 mol H2 (mol Pt min)−1 , relatively high values for Pt-based nanocatalysts [16,20–23] and other core–shell metal@silica NPs (Table 1) [14,15,27–32] for the same reaction. The recycle stability is very important for practical application of catalysts. As shown in Fig. 4b, there is no obvious decrease in catalytic activity after five runs, due to that the metal core NPs are protected by the silica shells, consistent with the results of TEM (Fig. 1d). The morphology and size are unchanged during the whole reactions. However, in the presence of Pt/SiO2 and Pt NPs, the catalytic activity shows a significant decrease after two runs (Fig. S5), probably due to that the Pt NPs without the silica shells protecting are easily aggregated (Fig. S1) and then resulting in lose of the activity site. In addition, the SEM image of the Pt@SiO2 (Fig. S6) after calcination indicates that the morphology and size of the sample are unchanged. The catalytic performance of Pt@SiO2 remained almost unchanged after the heat treatment (673 K), as shown in Fig. S7, which further displays a superior stability of this core–shell structured catalyst. As shown in Fig. S8a, it shows the plots of the hydrogen generated versus time during the catalytic hydrolysis of AB in the presence of different catalyst concentrations. The hydrogen generation rate (k) was determined from the linear portion of each plot. Fig. S8b shows the plot of hydrogen generation rate versus initial concentration of platinum, both in logarithmic scale. The straight line with a slope of 1.16 indicates that hydrolysis of AB is first order with respect to the catalyst concentration. The hydrogen generation rate (k) increases by increasing catalyst concentration as expected (i.e., lnk ≈ ln[Pt@SiO2 ] + 3.62). The effect of AB substrate concentration on the hydrogen generation rate was also studied by carrying out a series of experiments starting with different initial concentrations of AB while keeping the catalyst concentration
Table 1 The values of turnover frequency (TOF) for hydrolysis of AB catalyzed by different core–shell structured catalysts. Catalyst
Temperature (K)
Metal/AB (molar ratio)
TOF (mol H2 mol metal−1 min−1 )
Ref.
Ru@SiO2 Pt@SiO2 Ni@SiO2 Co@SiO2 Co-SiO2 Cu@SiO2 Hollow Ni-SiO2 Au-Ni@SiO2 Au-Co@SiO2
298 298 298 298 298 298 298 298 298
0.0025 0.0024 0.0425 0.024 0.05 0.09 0.05 0.084 0.065
200 158.6 18.5 13.3 12 3.2 2.7 2.5 0.5
[15] This work [27] [28] [29] [30] [31] [32] [14]
constant at room temperature, as shown in Fig. S9a. The straight line with a slope of 0.04 (Fig. S9b) indicates that hydrolysis of AB is zero order with respect to the substrate concentration. The hydrogen generation rate (k) is constant for different AB concentrations (i.e., lnk ≈ 2). In addition, it was found that the hydrogen generation rate rises substantially when the temperature increased from 298 K to 313 K (Fig. S10). 4. Conclusions In summary, highly active core–shell structured Pt@SiO2 NPs were successfully prepared at room temperature. It was found that the as-synthesized catalyst of Pt@SiO2 exhibits an excellent catalytic performance and long-term stability for the hydrogen generation from AB hydrolysis. After recycle test or heat treatment with high temperature, the morphology and catalytic performance of the sample remained almost unchanged. Acknowledgements This work was financially supported by National Natural Science Foundation of China (No. 21463012), Key Technology R&D Program of Jiangxi Province (No. 20114ACB01200) and Open Project Program of Key Laboratory of Functional Small Organic Molecule, Ministry of Education, Jiangxi Normal University (No. KLFS-KF201427). Z.-H. Lu was supported by the Young Scientist Foundation of Jiangxi Province (20133BCB23011) and “Gan-po talent 555” Project of Jiangxi Province. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc. 2015.02.094. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
A. Staubitz, A. Robertson, L. Manners, Chem. Rev. 110 (2010) 4079–4124. H.L. Jiang, Q. Xu, Catal. Today 170 (2011) 56–63. Z.H. Lu, Q. Xu, Funct. Mater. Lett. 5 (2012), 1230001. U. Sanyal, U.B. Demirci, B.R. Jagirdar, P. Miele, ChemSusChem 4 (2011) 1731–1739. M. Chandra, Q. Xu, J. Power Sources 156 (2006) 190–194. F.Y. Cheng, H. Ma, Y.M. Li, J. Chen, Inorg. Chem. 46 (2007) 788–794. Z.H. Lu, Q.L. Yao, Z.J. Zhang, Y.W. Yang, X.S. Chen, J. Nanomater. (2014) 729029. O. Metin, S. Duman, M. Dinc, S. Ozkar, J. Phys. Chem. C 115 (2011) 10736–10743. C.F. Yao, L. Zhuang, Y.L. Cao, X.P. Ai, H.X. Yang, Int. J. Hydrog. Energy 33 (2008) 2462–2467. J.M. Yan, Z.L. Wang, H.L. Wang, Q. Jiang, J. Mater. Chem. 22 (2012) 10990–10993. Y.W. Yang, Z.H. Lu, Y.J. Hu, Z.J. Zhang, W.M. Shi, X.S. Chen, T.T. Wang, RSC Adv. 4 (2014) 13749–13752. G.Z. Chen, S. Desinan, R. Nechache, R. Rosei, F. Rosei, D.L. Ma, Chem. Commun. 47 (2011) 6308–6310. Y.Z. Chen, Q. Xu, S.H. Yu, H.L. Jiang, Small 11 (2015) 71–76.
Y. Hu et al. / Applied Surface Science 341 (2015) 185–189 [14] Z.H. Lu, H.L. Jiang, M. Yadav, K. Aranishi, Q. Xu, J. Mater. Chem. 22 (2012) 5065–5071. [15] Q.L. Yao, W.M. Shi, G. Feng, Z.H. Lu, X.L. Zhang, D.J. Tao, D.J. Kong, X.S. Chen, J. Power Sources 257 (2014) 293–299. [16] M. Chandra, Q. Xu, J. Power Sources 168 (2007) 135–142. [17] X. Wang, D. Liu, S. Song, H. Zhang, Chem. Commun. 48 (2012) 10207–10209. [18] W.Y. Chen, J. Ji, X.Z. Duan, G. Qian, P. Li, X.G. Zhou, D. Chen, W.K. Yuan, Chem. Commun. 50 (2014) 2142–2144. [19] A. Aijaz, A. Karkamkar, Y.J. Choi, N. Tsumori, E. Ronnebro, T. Autrey, H. Shioyama, Q. Xu, J. Am. Chem. Soc. 134 (2012) 13926–13929. [20] X.J. Yang, F.Y. Cheng, Z.L. Tao, J. Chen, J. Power Sources 196 (2011) 2785–2789. [21] X. Wang, D.P. Liu, S.Y. Song, H.J. Zhang, Chem - Eur. J. 19 (2013) 8082–8086. [22] X.J. Yang, F.Y. Cheng, J. Liang, Z.L. Tao, J. Chen, Int. J. Hydrog. Energy 34 (2009) 8785–8791. [23] M. Wen, S.Q. Zhou, Q.S. Wu, J.Y. Zhang, Q.N. Wu, C.X. Wang, J. Power Sources 232 (2013) 86–92.
189
[24] L. Yang, N. Cao, C. Du, H.M. Dai, K. Hu, W. Luo, Mater. Lett. 115 (2014) 113–116. [25] A. Wootsch, C. Descorme, S. Rousselet, D. Duprez, C. Templier, Appl. Surface Science 253 (2006) 1310–1322. [26] G.Y. Zhao, H.L. Li, Appl. Surface Science 254 (2008) 3232–3235. [27] H. Liu, C.Y. Cao, P. Li, Y. Yu, W.G. Song, J. Energy Chem. 23 (2014) 50–56. [28] Ö. Metin, M. Dinc, Z.S. Eren, S. Özkar, Int. J. Hydrog. Energy 36 (2011) 11528–11535. [29] T. Umegaki, J.M. Yan, X.B. Zhang, H. Shioyama, N. Kuriyama, Q. Xu, J. Power Sources 195 (2010) 8209–8214. [30] Q.L. Yao, Z.H. Lu, Z.J. Zhang, X.S. Chen, Y.Q. Lan, Sci. Rep. 4 (2014) 7597. [31] T. Umegaki, J.M. Yan, X.B. Zhang, H. Shioyama, N. Kuriyama, Q. Xu, J. Power Sources 191 (2009) 209–216. [32] H.L. Jiang, T. Umegaki, T. Akita, X.B. Zhang, M. Haruta, Q. Xu, Chem. - Eur. J. 16 (2010) 3132–3137.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Core–shell nanospheres Pt@SiO2 for catalytic hydrogen production Yujuan Hu, Yuqing Wang, Zhang-Hui Lu ∗ , Xiangshu Chen ∗∗ , Lihua Xiong Jiangxi Inorganic Membrane Materials Engineering Research Centre, Key Laboratory of Functional Small Organic Molecule (Ministry of Education), College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang 330022, China
a r t i c l e
i n f o
Article history: Received 17 November 2014 Received in revised form 13 February 2015 Accepted 14 February 2015 Available online 4 March 2015 Keywords: Core–shell nanoparticles Hydrolysis Ammonia borane Hydrogen
a b s t r a c t Ultrafine platinum nanoparticles (NPs) embedded in silica nanospheres (Pt@SiO2 ) have been synthesized in a NP-5/cyclohexane reversed-micelle system followed by NaBH4 reduction. The as-synthesized core–shell nanocatalysts Pt@SiO2 were characterized by scanning electron microscopy, transmission electron microscopes, X-ray powder diffraction analysis, energy dispersive X-ray spectrometer and nitrogen adsorption–desorption investigations. Interestingly, the as-synthesized core–shell nanocatalysts Pt@SiO2 showed an excellent catalytic performance in hydrogen generation from the hydrolysis of ammonia borane (BH3 NH3 , AB) at room temperature. Especially, the catalytic performance of the Pt@SiO2 remained almost unchanged after the five recycles and even after the heat treatment (673 K), because the silica shells inhibit aggregation or deformation of the metal cores. Besides, the kinetic studies showed that the catalytic hydrolysis of AB was first order with respect to the catalyst concentration and zero order with respect to the substrate concentration, respectively. The excellent catalytic activity and stability of Pt@SiO2 can make it have a bright future in the practical application. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, hydrogen is one of the most attractive energy carriers for a long-term solution to current energy problems owing to its high calorific value and non-polluting nature. The safe and efficient storage of hydrogen is of great significance to fulfill the goal of hydrogen-powered transport systems. In recent years, ammonia borane (NH3 BH3 , AB) has drawn much attention as a good candidate for hydrogen reservoir due to its high hydrogen capacity of 19.6 wt%, good stability and non-toxicity [1–4]. Catalytic hydrolysis of AB in the presence of a suitable catalyst can release three mol of hydrogen per mol of AB at ambient temperature, which presents a high hydrogen capacity up to 9.0 wt% of the starting materials (AB and H2 O), thus making itself an effective approach for the release of hydrogen stored in AB [5–11]. On this count, it is the key point to select a suitable catalyst. A number of catalysts have been tested in hydrogen generation from the hydrolysis of AB, including transition metal NPs such as platinum, rhodium, ruthenium, palladium, cobalt and nickel NPs [2,3]. Although rhodium, platinum and ruthenium metal NPs have high price and limited abundance, they are superior to the non-noble metal NPs due to their long lifetime and high
activity in the hydrolysis of AB. Therefore the use of noble metal NPs as catalysts has been intensively studied in recent years [12–24]. However, big challenges still remain. To make the AB complex a practical hydrogen reservoir for “on board” applications, highly efficient and stable catalysts are highly desired. Platinum-based catalysts have been attracted much attention in the past decades as the excellent and versatile catalysts in various important reactions [16–23,25,26]. Taking into account the practical applications of this system, various solid supports, such as silica, carbon, metal oxide and zeolite, have been employed to achieve high Pt utilization through preparing highly dispersed Pt NPs. However, the aggregation of metal particles accompanied by the loss of catalytic activity is still a great challenge. The encapsulation of metal particles into the supports with core–shell-type structures has attracted great attention owing to its high stability [14,15,27–32]. Herein, we report a one-pot synthesis of core–shell nanospheres Pt@SiO2 , which show high catalytic performance and excellent recycle stability for the hydrogen generation from AB hydrolysis at room temperature.
2. Experimental 2.1. Chemicals
∗ Corresponding author. Tel.: +86 791 88121974; fax: +86 791 88121974. ∗∗ Corresponding author. E-mail addresses: [email protected] (Z.-H. Lu), [email protected] (X. Chen). http://dx.doi.org/10.1016/j.apsusc.2015.02.094 0169-4332/© 2015 Elsevier B.V. All rights reserved.
Potassium tetrachloroplatinate (II) (K2 PtCl4 , J&K, 99.95%), tetraethoxysilane (TEOS, Sigma–Aldrich, 98%), polyethylene
186
Y. Hu et al. / Applied Surface Science 341 (2015) 185–189
glycolmono-4-nonylphenyl ether n ≈ 5 (NP-5, Tokyo Chemical Industry Co., Ltd), ammonia solution (NH3 ·H2 O, Nanchang Chemical Works, 28%), cyclohexane (C6 H12 , Tianjin Fuchen Chemical Reagent, >99.5%), methanol (CH3 OH, Tianjin Fuchen Chemical Reagent, 99.5%), and acetone ((CH3 )2 CO, Nanchang Chemical Works, >99.5%) were used as received. Ammonia borane (NH3 BH3 , 90%) and sodium borohydride (NaBH4 , 99%) were purchased from Aldrich. All chemicals were used without any purification. Ultrapure deionized water with the specific resistance of 18.3 M cm was generated by reversed osmosis followed by ion exchange and filtration. 2.2. Synthesis of Pt@SiO2 The reverse micelle system used in this study composed of water, surfactant and organic solvent. 2.16 mL of aqueous K2 PtCl4 solution (containing 74.75 mg K2 PtCl4 , corresponding to 5 wt% Pt loading) was added into 480 mL of NP-5 (20.16 g)/cyclohexane solution, and the mixture was stirred at room temperature for 15 h. Next, 2.16 mL of ammonium hydroxide solution was injected rapidly into the mixture under continuously stirring and after 2 h TEOS (2.49 mL) was added. After two days of stirring at room temperature, the as-synthesized silica nanospheres were isolated by adding methanol followed by washing with cyclohexane and acetone to remove surfactant and unreacted chemicals, and centrifugation at 10,000 rpm for 8 min. Then, the as-synthesized Pt@SiO2 NPs were reduced by 20 mL of freshly prepared solution of NaBH4 (100 mg). Finally, the solid Pt@SiO2 NPs were received by drying the samples in vacuum oven at 40 ◦ C for overnight. In order to test the thermally stable of Pt@SiO2 , the as-synthesized NPs were calcined at 400 ◦ C for 2 h. 2.3. Synthesis of Pt/SiO2 The Pt/SiO2 NPs in this study were prepared by the conventional impregnation. The Pt content was kept to be about 5 wt%. 1.01 mg K2 PtCl4 and 19 mg SiO2 were dissolved in 10 mL of distilled water for 2 h. Then, the sample was reduced by 5 mg NaBH4 . Finally, the received Pt/SiO2 NPs were used to catalytic hydrolysis of AB (34.29 mg). 2.4. Synthesis of Pt NPs 1.01 mg K2 PtCl4 was dissolved in 10 mL of distilled water and 5 mg NaBH4 was added into the solution. Then, the received Pt NPs were used to catalytic hydrolysis of AB (34.29 mg).
2.6. Catalytic hydrolysis of AB by Pt@SiO2 (C 400) NPs The catalytic activity of Pt@SiO2 (C 400) NPs toward AB hydrolysis was determined by measuring the rate of hydrogen generation in a typical experiment described in Section 2.5. The catalytic performance of Pt@SiO2 has no significant changed after the heat treatment with 400 ◦ C. 2.7. Kinetic study of Pt@SiO2 in hydrolysis of AB In order to establish the rate law for the hydrolytic dehydrogenation of AB catalyzed by Pt@SiO2 , two different sets of experiment were performed in the same way as described in Section 2.5. In the first set of experiments, keeping the initial AB concentration constant at 100 mM, the amount of Pt@SiO2 catalyst (5 wt%) was varied in 0.082 mM, 0.163 mM, 0.245 mM and 0.326 mM at 25 ◦ C. In the second set of experiments, keeping the initial Pt@SiO2 concentration (5 wt%) constant at 0.245 mM, the amount of AB was varied in 100 mM, 150 mM, 200 mM and 300 mM at 25 ◦ C. 2.8. Recycle test of Pt@SiO2 in hydrolysis of AB The reactions were repeated five times under ambient atmosphere at room temperature. After hydrolysis reaction test, the catalysts were separated from the reaction solution by centrifugation, washed with water for three times and dried in vacuum oven at 40 ◦ C overnight. 2.9. Characterization The size and morphology composition of the samples were characterized by field emission scanning electron microscopy (FE-SEM, Hitachi, SU8020) at acceleration voltages of 3–10 kV, field emission transmission electron microscope (FE-TEM, FEI, Tecnai G2 F20 (200 kV)) and an energy dispersive X-ray spectrometer (EDS) attached on FE-TEM for elemental analysis. The TEM samples were prepared by depositing one or two droplets of the nanoparticle suspensions on to the amorphous carbon coated copper grids. The structures were characterized by Powder X-ray diffraction (XRD, Rigaku RINT-2200 X-ray diffractometer with a CuKa source (40 kV, 20 mA)) and X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI, USA) on a K-alpha spectrometer with monochromatic Al K␣ radiation (hv = 1486.6 eV). Brunauer–Emmett–Teller (BET, BEL Japan, Inc., BELSORP-mini II) surface area and pore parameters analysis were characterized by N2 adsorption–desorption. 3. Results and discussion 3.1. Characterization of the samples
2.5. Catalytic hydrolysis of AB by as-prepared Pt@SiO2 In a typical experiment, the catalytic activity of Pt@SiO2 NPs toward AB hydrolysis was determined by measuring the rate of hydrogen generation. 9.546 mg of Pt@SiO2 NPs and 10 mL of deionized water were added into a two-necked round-bottomed flask, which was kept in a water bath heating at 25 ◦ C. A gas buret was connected to one neck of the flask to measure the volume of hydrogen, and 34.29 mg of AB was introduced into the reaction flask through the other neck. The reaction was started when AB was added into the flask under electric stirring at room temperature. The evolution of hydrogen was monitored using the gas buret. The hydrolysis of NH3 BH3 can be briefly expressed as follows: NH3 BH3 + 2H2 O → NH4 + + BO2 − + 3H2 . The volume of released hydrogen gas was monitored by recording time for the displacement of water. The reaction was completed when there was no more hydrogen gas generated.
The as-synthesized NPs were characterized by SEM and TEM. As shown in Fig. 1a, SEM image shows that the size of the assynthesized Pt@SiO2 is quite uniform with an average diameter of ∼25 nm. The monodisperse spherical morphology of Pt@SiO2 can be further confirmed by the TEM image (Fig. 1b). It can be seen from Fig. 1b that the single Pt NP as a core with a diameter of ∼4 nm is effectively embedded into the silica nanosphere (∼25 nm). However, the large agglomerate/aggregate Pt NPs were observed in Pt/SiO2 (Pt with an average diameter of ∼16 nm) and Pt NPs in Fig. S1 (supplementary data), which may lead to a decrease in the catalytic activity and reusability. The high-resolution TEM (HRTEM) image (Fig. 1c) of Pt@SiO2 demonstrates the polycrystalline structure of the Pt core with fringes associated with a d-spacing of ∼0.22 nm, corresponding to the (1 1 1) plane of the Pt face-centercubic (fcc) structure. The EDS of Pt@SiO2 shows that the sample includes elements Pt, Si, O and Cu which is from TEM grids in
Y. Hu et al. / Applied Surface Science 341 (2015) 185–189
187
Fig. 1. (a) SEM, (b) TEM, (c) HRTEM images of Pt@SiO2 and (d) TEM of Pt@SiO2 after five recycles.
Fig. 2. (a) EDS and (b) XRD of Pt@SiO2 .
Fig. 2a. The X-ray diffraction (XRD) patterns of the samples before and after hydrolysis of AB are given in Fig. 2b. The strong and broad diffraction peak at 2 = 15–35◦ observed in the XRD patterns is attributed to the amorphous silica. And other diffraction peaks in XRD patterns could be indexed to those of Pt fcc structure (JCPDS No. 01-1194). A nitrogen adsorption–desorption study (Fig. 3 and Fig. S3) shows that the Pt@SiO2 , Pt/SiO2 and Pt NPs have a Brunauer–Emmett–Teller (BET) surface area of 104, 118 and 3 m2 g−1 , respectively. The isotherms of Pt@SiO2 NPs exhibit a type IV isotherm with a hysteresis loop at a P/P0 region of 0.8–1.0 (Fig. 3), indicating the existence of mesopores in the sample, confirmed by the pore size (0.48 cm3 g−1 ) distribution of core–shell sample (Fig. S2). The XPS pattern shows the significant Pt 4f signal corresponding to the binding energy of Pt in Fig. S4. The Pt 4f7/2 and Pt 4f5/2 binding energies correspond to Pt (0).
in Fig. 4a. The evolution of hydrogen is finished in 7.72, 55.98 and 111 min, respectively, in the presence of the as-synthesized Pt@SiO2 , Pt/SiO2 and Pt NPs at room temperature. The hydrogen generation rates from AB are in the order of Pt@SiO2 > Pt/SiO2 > Pt
3.2. Catalytic properties Catalytic activities of Pt@SiO2 NPs together with Pt, SiO2 and Pt/SiO2 (Pt on Silica) have been studied for the hydrolytic dehydrogenation of AB at room temperature, and the results are show
Fig. 3. The nitrogen adsorption-desorption isotherms of Pt@SiO2 .
188
Y. Hu et al. / Applied Surface Science 341 (2015) 185–189
Fig. 4. (a) Plots of the volume of hydrogen generation from AB (100 mM, 10 mL) hydrolysis as a function of time catalyzed by Pt@SiO2 , Pt/SiO2 , Pt and SiO2 at 25 ◦ C, respectively. (b) Recyclability of Pt@SiO2 .
with the same Pt content while the particle sizes of these catalysts are in the order of Pt@SiO2 < Pt/SiO2 < Pt (Fig. 1 and Fig. S1). Among all of the catalysts, Pt@SiO2 presents the highest catalytic activity to release a stoichiometric amount of hydrogen from aqueous AB (H2 /AB = 3.0) with a turnover frequency (TOF) value of 158.6 mol H2 (mol Pt min)−1 , relatively high values for Pt-based nanocatalysts [16,20–23] and other core–shell metal@silica NPs (Table 1) [14,15,27–32] for the same reaction. The recycle stability is very important for practical application of catalysts. As shown in Fig. 4b, there is no obvious decrease in catalytic activity after five runs, due to that the metal core NPs are protected by the silica shells, consistent with the results of TEM (Fig. 1d). The morphology and size are unchanged during the whole reactions. However, in the presence of Pt/SiO2 and Pt NPs, the catalytic activity shows a significant decrease after two runs (Fig. S5), probably due to that the Pt NPs without the silica shells protecting are easily aggregated (Fig. S1) and then resulting in lose of the activity site. In addition, the SEM image of the Pt@SiO2 (Fig. S6) after calcination indicates that the morphology and size of the sample are unchanged. The catalytic performance of Pt@SiO2 remained almost unchanged after the heat treatment (673 K), as shown in Fig. S7, which further displays a superior stability of this core–shell structured catalyst. As shown in Fig. S8a, it shows the plots of the hydrogen generated versus time during the catalytic hydrolysis of AB in the presence of different catalyst concentrations. The hydrogen generation rate (k) was determined from the linear portion of each plot. Fig. S8b shows the plot of hydrogen generation rate versus initial concentration of platinum, both in logarithmic scale. The straight line with a slope of 1.16 indicates that hydrolysis of AB is first order with respect to the catalyst concentration. The hydrogen generation rate (k) increases by increasing catalyst concentration as expected (i.e., lnk ≈ ln[Pt@SiO2 ] + 3.62). The effect of AB substrate concentration on the hydrogen generation rate was also studied by carrying out a series of experiments starting with different initial concentrations of AB while keeping the catalyst concentration
Table 1 The values of turnover frequency (TOF) for hydrolysis of AB catalyzed by different core–shell structured catalysts. Catalyst
Temperature (K)
Metal/AB (molar ratio)
TOF (mol H2 mol metal−1 min−1 )
Ref.
Ru@SiO2 Pt@SiO2 Ni@SiO2 Co@SiO2 Co-SiO2 Cu@SiO2 Hollow Ni-SiO2 Au-Ni@SiO2 Au-Co@SiO2
298 298 298 298 298 298 298 298 298
0.0025 0.0024 0.0425 0.024 0.05 0.09 0.05 0.084 0.065
200 158.6 18.5 13.3 12 3.2 2.7 2.5 0.5
[15] This work [27] [28] [29] [30] [31] [32] [14]
constant at room temperature, as shown in Fig. S9a. The straight line with a slope of 0.04 (Fig. S9b) indicates that hydrolysis of AB is zero order with respect to the substrate concentration. The hydrogen generation rate (k) is constant for different AB concentrations (i.e., lnk ≈ 2). In addition, it was found that the hydrogen generation rate rises substantially when the temperature increased from 298 K to 313 K (Fig. S10). 4. Conclusions In summary, highly active core–shell structured Pt@SiO2 NPs were successfully prepared at room temperature. It was found that the as-synthesized catalyst of Pt@SiO2 exhibits an excellent catalytic performance and long-term stability for the hydrogen generation from AB hydrolysis. After recycle test or heat treatment with high temperature, the morphology and catalytic performance of the sample remained almost unchanged. Acknowledgements This work was financially supported by National Natural Science Foundation of China (No. 21463012), Key Technology R&D Program of Jiangxi Province (No. 20114ACB01200) and Open Project Program of Key Laboratory of Functional Small Organic Molecule, Ministry of Education, Jiangxi Normal University (No. KLFS-KF201427). Z.-H. Lu was supported by the Young Scientist Foundation of Jiangxi Province (20133BCB23011) and “Gan-po talent 555” Project of Jiangxi Province. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc. 2015.02.094. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
A. Staubitz, A. Robertson, L. Manners, Chem. Rev. 110 (2010) 4079–4124. H.L. Jiang, Q. Xu, Catal. Today 170 (2011) 56–63. Z.H. Lu, Q. Xu, Funct. Mater. Lett. 5 (2012), 1230001. U. Sanyal, U.B. Demirci, B.R. Jagirdar, P. Miele, ChemSusChem 4 (2011) 1731–1739. M. Chandra, Q. Xu, J. Power Sources 156 (2006) 190–194. F.Y. Cheng, H. Ma, Y.M. Li, J. Chen, Inorg. Chem. 46 (2007) 788–794. Z.H. Lu, Q.L. Yao, Z.J. Zhang, Y.W. Yang, X.S. Chen, J. Nanomater. (2014) 729029. O. Metin, S. Duman, M. Dinc, S. Ozkar, J. Phys. Chem. C 115 (2011) 10736–10743. C.F. Yao, L. Zhuang, Y.L. Cao, X.P. Ai, H.X. Yang, Int. J. Hydrog. Energy 33 (2008) 2462–2467. J.M. Yan, Z.L. Wang, H.L. Wang, Q. Jiang, J. Mater. Chem. 22 (2012) 10990–10993. Y.W. Yang, Z.H. Lu, Y.J. Hu, Z.J. Zhang, W.M. Shi, X.S. Chen, T.T. Wang, RSC Adv. 4 (2014) 13749–13752. G.Z. Chen, S. Desinan, R. Nechache, R. Rosei, F. Rosei, D.L. Ma, Chem. Commun. 47 (2011) 6308–6310. Y.Z. Chen, Q. Xu, S.H. Yu, H.L. Jiang, Small 11 (2015) 71–76.
Y. Hu et al. / Applied Surface Science 341 (2015) 185–189 [14] Z.H. Lu, H.L. Jiang, M. Yadav, K. Aranishi, Q. Xu, J. Mater. Chem. 22 (2012) 5065–5071. [15] Q.L. Yao, W.M. Shi, G. Feng, Z.H. Lu, X.L. Zhang, D.J. Tao, D.J. Kong, X.S. Chen, J. Power Sources 257 (2014) 293–299. [16] M. Chandra, Q. Xu, J. Power Sources 168 (2007) 135–142. [17] X. Wang, D. Liu, S. Song, H. Zhang, Chem. Commun. 48 (2012) 10207–10209. [18] W.Y. Chen, J. Ji, X.Z. Duan, G. Qian, P. Li, X.G. Zhou, D. Chen, W.K. Yuan, Chem. Commun. 50 (2014) 2142–2144. [19] A. Aijaz, A. Karkamkar, Y.J. Choi, N. Tsumori, E. Ronnebro, T. Autrey, H. Shioyama, Q. Xu, J. Am. Chem. Soc. 134 (2012) 13926–13929. [20] X.J. Yang, F.Y. Cheng, Z.L. Tao, J. Chen, J. Power Sources 196 (2011) 2785–2789. [21] X. Wang, D.P. Liu, S.Y. Song, H.J. Zhang, Chem - Eur. J. 19 (2013) 8082–8086. [22] X.J. Yang, F.Y. Cheng, J. Liang, Z.L. Tao, J. Chen, Int. J. Hydrog. Energy 34 (2009) 8785–8791. [23] M. Wen, S.Q. Zhou, Q.S. Wu, J.Y. Zhang, Q.N. Wu, C.X. Wang, J. Power Sources 232 (2013) 86–92.
189
[24] L. Yang, N. Cao, C. Du, H.M. Dai, K. Hu, W. Luo, Mater. Lett. 115 (2014) 113–116. [25] A. Wootsch, C. Descorme, S. Rousselet, D. Duprez, C. Templier, Appl. Surface Science 253 (2006) 1310–1322. [26] G.Y. Zhao, H.L. Li, Appl. Surface Science 254 (2008) 3232–3235. [27] H. Liu, C.Y. Cao, P. Li, Y. Yu, W.G. Song, J. Energy Chem. 23 (2014) 50–56. [28] Ö. Metin, M. Dinc, Z.S. Eren, S. Özkar, Int. J. Hydrog. Energy 36 (2011) 11528–11535. [29] T. Umegaki, J.M. Yan, X.B. Zhang, H. Shioyama, N. Kuriyama, Q. Xu, J. Power Sources 195 (2010) 8209–8214. [30] Q.L. Yao, Z.H. Lu, Z.J. Zhang, X.S. Chen, Y.Q. Lan, Sci. Rep. 4 (2014) 7597. [31] T. Umegaki, J.M. Yan, X.B. Zhang, H. Shioyama, N. Kuriyama, Q. Xu, J. Power Sources 191 (2009) 209–216. [32] H.L. Jiang, T. Umegaki, T. Akita, X.B. Zhang, M. Haruta, Q. Xu, Chem. - Eur. J. 16 (2010) 3132–3137.