Volume 9 Issue 9 - July 24, 2009 PDF
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Simple and fast fabrication of polymer template-Ru composite as a catalyst for hydrogen generation from alkaline NaBH4 solution
Chan-Li Hsueh1, Chuh-Yung Chen2,*, Jie-Ren Ku1, Shing-Fen Tsai1, Ya-Yi Hsu1, Fanghei Tsau1, Ming-Shan Jeng1

1Energy and Environment Research Laboratories (EEL), Industrial Technology Research Institute (ITRI), Hsinchu, Taiwan
2Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan
ccy7@ccmail.ncku.edu.tw

Journal of Power Sources, 177/2, 485-492, March 2008.

 
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Introduction

Most of the oil daily extracted is consumed in transportation (approximately 66.6% in America) [1]. In these environment therefore, car emissions represents the main pollution source. Hydrogen cars, by eliminating CO2 emissions, could drastically reduce in short times pollution, especially in our living areas. The main obstacle, hindering the introduction into the market of hydrogen cars, is represented by the low efficiency of hydrogen storage system. We can assume that a fuel cell equipped car would run about 100 km per kg of hydrogen burned. Consequently a standard of 400 km would be fulfilled by 4 kg of hydrogen. Fig.1 shows various storage systems dimensioned to deliver 4 kg of hydrogen. Among the different storage systems, the hydrolysis of hydride seems the best promising one [2]
Figure 1. Energy density, both volumetric and gravimetric, of various systems for hydrogen storage. The 2010 and 2015 DOE targets correspond to 6.0 and 9.0 % wt[2].

Among the chemical hydrides, NaBH4 is much more attractive due to its various advantages: relatively high hydrogen content (10.7 wt.%); stable and nonflammable alkaline solution; controllable hydrolysis reaction; environmentally friendly and renewable. When contacting with a given catalyst, its alkaline solutions (the alkali usually acts as a stabilizer to restrain the self-hydrolysis of NaBH4 in aqueous solution) can release hydrogen rapidly in the following way:
(1)

This hydrolysis reaction proceeds at various rates that depend on the catalyst and its method of preparation. In this study a simple and fast method for fabricating a polymer template-Ru composite as a catalyst, Ru/IR-120, was developed for hydrolyzing sodium borohydride from its alkaline solution.

Experimental

A weighed amount of Amberlite IR-120 cationic exchange resin beads was added to the RuCl3 solution that has been stirred for 1 hr at the ambient temperature. After chelating with Ru3+, Amberlite IR-120 surface structure transformed from RZSO3––H+ (RZ = polymer matrix of the resin) into (RZSO3)3–Ru3+ (Ru3+/IR-120). The resins were carefully washed repeatedly using deionized water to remove the soluble ions and then mixed with NaBH4 solution as a reducing agent. Then, the resins were filtrated and washed repeatedly with deionized water and vacuum-dried at 90 ℃ to eliminate residual water and hydrogen. Finally, a polymer template-Ru composite (Ru/IR-120) catalyst was obtained. The catalytic activity of Ru/IR-120 catalyst was determined by measuring the amount of hydrogen that was generated by the hydrolysis of sodium borohydride. The NaBH4 solution was thermostatically maintained at a preset temperature in the sealed flask, in which the solution was stirred vigorously. A graduated glass column filled with water was connected to the top outlet of the flask as a gas burette. The volume of hydrogen gas evolved was measured from the water level change in the column.

Results and discussion

Figure 2 displays photographs of the fabrication of the catalyst. Figure 2a shows the original Amberlite IR-120 cationic exchange resin beads. The surface color of the original resin beads is yellowish-brown. After the original resin beads had been chelated with Ru3+, the surface color turned from yellowish-brown to black (Ru3+/IR-120), as presented in Fig. 2b. The surface color of Ru3+/IR-120 became silvery white (Fig. 2c) immediately upon contact between the reducing agent and Ru3+/IR-120. Figure 3 presents SEM micrographs of IR-120, Ru3+/IR-120 and Ru/IR-120. Figures 3a and 3b indicate that the IR-120 and Ru3+/IR-120 surfaces are smooth under the highly magnified SEM images (50000x). However, after reduction reaction, Ru nanoparticles aggregate on the Ru/IR-120 surface (Fig. 3c). Figure 4 presents the EDS of the original IR-120 resin beads (curve a) and that of the Ru/IR-120 catalyst (curves b). Curve a reveals that C, O and S are the dominant elements detected on the surface of IR-120. However, curve b indicates that only Ru was added to the surface of IR-120 after reduction. Table 1 presents the quantitative surface chemical compositions of the Ru/IR-120 catalyst, determined by EDS. Therefore, TGA measurements were made to quantify the amount of Ru in the Ru/IR-120 catalyst. Figure 5 plots the TGA curves of both IR-120 and Ru/IR-120. According to the TGA curves, the Ru/IR-120 catalyst yields a 1% final residue, while the original IR-120 resin yields 0%. The residual percentage of the Ru/IR-120 confirms the presence of Ru. The Amberlite IR-120 resin is a strong acidic cation exchange resin. Sodium borohydride hydrolysis can be accelerated not only by catalysts but also by acid [4]. In order to understand the catalysis of IR-120 resin for the generation of H2 from alkaline NaBH4 solution, a comparison of catalysis efficacy between IR-120 resin and Ru/IR-120 was conducted. The reactions were carried out under the following conditions: 5 wt.% NaBH4 + 1 wt.% NaOH solution at 25℃. As Fig. 6 reveals, the effect of IR-120 resin on hydrogen generation was considerably smaller than that of Ru/IR-120. The catalysis effect of IR-120 resin for hydrogen generation from alkaline NaBH4 solution is therefore neglected in this study. Figure 7 plots H2 generation rates of 5 wt.% NaBH4 +1 wt.% NaOH (and 94 wt% water solutions) at various temperatures in the 5-55℃ range. As expected, the H2 generation rate increases as the temperature increases. The comparisons of H2 generation performances of Ru/IR-120 catalyst with those of other catalysts are listed in Table 2. The H2 generations rate of Ru/IR-120 catalyst is smaller than IRA-400 supported Ru or γ-Al2O3supported Co. However, the Ru loading in the catalysts used in this paper is only 1 wt.%.
Figure 2. Photographs of (a) IR-120; (b) Ru3+/IR-120 and (c)Ru/IR-120.
Figure 3. Scanning electron micrographs of (a) IR-120; (b) Ru3+/IR-120 and (c) Ru/IR-120
Figure 5. TGA curves of (a) IR-120 and (b) Ru/IR-120.
Figure 4. EDS spectra of (a) IR-120 and         (b) Ru/IR-120.

Figure 7. Volume of hydrogen generated as a function of time in different solution temperatures (5 wt.% NaBH4 + 1 wt.% NaOH solution, 200mg Ru/IR-120 catalyst).
Table 1 Surface weight and atomic percentage of the elements present in polymer template (Amberlite IR-120 resin) and catalyst (Ru/IR-120) by EDS
Table 2 Comparisons of H2 generation performance between various catalysts

Conclusion

This research focuses on the simple and fast method for fabricating a polymer template-Ru composite as a catalyst, Ru/IR-120, was developed for hydrolyzing sodium borohydride from its alkaline solution. This method is suitable for the preparation of not only polymer template-Ru composite but also other polymer template-metal composites, such as Co, Ni, Fe and Cu. A Ru loading as low as 1wt.% in the Ru/IR-120 was achieved using this technique. When a Ru/IR-120 catalyst that contains 1 wt.% Ru was used, a maximum hydrogen generation rate of 132 ml min−1 g−1 from 5 wt.% NaBH4 + 1 wt.% NaOH solution at 25 ℃ was obtained. There is still room for improvement for the dispersion of Ru nanoparticles on IR-120 surface, which would markedly improve the hydrogen generation rate further. Moreover, as the solution temperature increases, the hydrogen generation rate increases.

Reference

[1] http://www.bts.gov/publications/national_transportation_statistics/html/table_04_03.html
[2] http://www.worldenergy.org/documents/p001360.doc
[3] R. S. Juang, T. S. Lee, J. Hazard. Mater. B92 (2002) 301–314.
[4] S. Özkar, M. Zahmakıran, J Alloys Compounds. 404-406 (2005) 728–731.
[5] S. C. Amendola, S. L. Sharp-Goldman, M. S. Janjua, N. C. Spencer, M. T. Kelly, P. J. Petillo, M. Binder, Int. J. Hydrog. Energy 25 (2000) 969-975.
[6] W. Ye, H. Zhang, D. Xu, L. Ma, B. Yi, J. Power Sources 164 (2007) 544–548.
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