In-Situ X-ray Diffraction Study of 纬-Mg(BH4)2 Decomposition

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• 作者：Mark Paskevicius ; Mark P. Pitt ; Colin J. Webb ; Drew A. Sheppard ; Uffe Fils酶 ; Evan MacA. Gray ; Craig E. Buckley
• 刊名：The Journal of Physical Chemistry C
• 出版年：2012
• 出版时间：July 26, 2012
• 年：2012
• 卷：116
• 期：29
• 页码：15231-15240
• 全文大小：617K
• 年卷期：v.116,no.29(July 26, 2012)
• ISSN：1932-7455

文摘

We have studied the complex decomposition mechanism of cubic 纬-Mg(BH₄)₂ (Ia3̅d, a = 15.7858(1) 脜) by in-situ synchrotron X-ray diffraction, temperature-programmed desorption, visual observation of the melt, and Fourier transform infrared (FTIR) spectroscopy. The decomposition and release of hydrogen proceeds through eight distinct steps, including two polymorphic transitions before melting, with a new 蔚-Mg(BH₄)₂ phase at ca. 150 掳C. After melting, strong changes in sample color from yellow to brown to gray are consistent with the unknown Mg鈥揃鈥揌 phase(s) (that diffract with high d-spacing halos) in the sample changing from an average composition of MgB₂H_5.3 at 325 掳C, to MgB_2.9H_3.2 at 350 掳C, and to MgB_4.0H_3.7 by 450 掳C. From 350 to 450 掳C, the crystalline Mg proportion increases. No combination of previously assigned anionic B_nH_m species (including MgB₁₂H₁₂ and Mg(B₃H₈)₂) can account for the average composition of the unknown proportion of the sample. This is supported by FTIR spectra showing an absence of terminal B鈥揌 resonances in the 2500 cm^鈥? region that are present for B₁₂H₁₂ and B₃H₈ anionic species. Our combined analysis strongly indicates the presence of as yet unidentified Mg鈥揃鈥揌 phase(s) in postmelted decomposed Mg(BH₄)₂ samples.