Improved dehydrogenation of TiF 3 -doped NaAlH 4 using ordered mesoporous SiO 2 as a codopant
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ngtao Li, Fang Fang, Guangyou Zhou, Xuebin Yu, Guorong Chen, and Dalin Suna) Department of Materials Science, Fudan University, Shanghai 200433, People’s Republic of China
Liuzhang Ouyang and Min Zhub) School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China (Received 8 April 2010; accepted 2 June 2010)
A study of the influence of mesoporous SiO2 on the dehydrogenation of NaAlH4 and TiF3-doped NaAlH4 revealed that the amount of hydrogen evolved is 3.8 wt% for the pristine NaAlH4 and around 4.2 wt% for the TiF3-doped NaAlH4, but increases to 4.9–5.0 wt% once the samples are doped with mesoporous SiO2 in the temperature range of 100–350 C. A favorable synergistic effect on the NaAlH4 dehydrogenation is achieved as mesoporous SiO2 is added as a codopant along with TiF3, which is associated with the nanosized pores and high specific surface area of mesoporous SiO2. The catalytic mechanism of mesoporous SiO2 is more physical than chemical relative to the catalytic mechanism of TiF3.
I. INTRODUCTION
Using hydrogen as an energy carrier offers the prospect of operating essentially free of pollutant and greenhouse gas emissions. To realize this goal, improved approaches for either stationary or on-board applications are needed for the high-capacity storage of hydrogen at temperatures ranging from near ambient to about 100 C and at pressures below about 10 MPa. These conditions favor storage based on the solid-state materials, rather than storage based on compressed or liquid hydrogen, which requires high pressures (70 MPa) or low temperatures (253 C), respectively. As a result, complex hydrides such as alanates (AlH4) and borohydrides (BH4) have recently attracted much interest because of their intrinsic high hydrogen contents.1–4 However, their practical applications are plagued by: (i) the kinetic barriers to dehydrogenation and/or rehydrogenation at a temperature below 120 C and (ii) the poor reversibility in the cyclic de-/rehydrogenation.5,6 These drawbacks arise mainly from their bonding chemistry, in which the hydrogen is covalently bound in the form of hydride anions.7 In the presence of Ti-based dopants, the twostep dehydrogenation [Eqs. (1) and (2)] of NaAlH4 can Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2010.0260 J. Mater. Res., Vol. 25, No. 10, Oct 2010
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be kinetically enhanced, with the released hydrogen of 3.7 and 1.8 wt%, respectively.8,9 1 2 NaAlH4 ⇄ Na3 AlH6 þ Al þ H2 3 3 1 1 1 Na3 AlH6 ⇄NaH þ Al þ H2 3 3 2
; :
ð1Þ ð2Þ
Moreover, re-hydrogenating the resulting NaH and Al phases back to the NaAlH4 can also be achieved under mild temperatures and pressures.10,11 Unfortunately, after repeated de-/rehydrogenation cycling (3–4 times), only 3.0–4.0 wt% hydrogen is left in a Ti-doped NaAlH4 system12 which is lower than the theoretical value of 5.5 wt% [see Eqs. (1) and (2)]. Thus, the positive be
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