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Issue 4, 2014
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Structure and thermal properties of composites with RE-borohydrides (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Er, Yb or Lu) and LiBH4

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Abstract

RECl3 (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Er, Yb, Lu) and LiBH4 ball-milled in the molar ratio of 1 : 6 resulted in formation of numerous rare-earth metal (RE) borohydrides and LiCl in mixtures with excess LiBH4. The reaction products were characterized by powder X-ray diffraction and infrared spectroscopy, and their hydrogen sorption properties were investigated by thermogravimetric analysis combined with differential scanning calorimetry and in situ synchrotron radiation powder X-ray diffraction. La, Ce, Pr and Nd form LiRE(BH4)3Cl compounds which crystallize in the cubic space group I[4 with combining macron]3m. Sm, Gd, Tb, Er and Yb form RE(BH4)3 phases crystallizing in the cubic space group Pa[3 with combining macron], with a possible polymorphic transition to a higher symmetry space group, Pm[3 with combining macron]m. The smaller RE-elements Yb and Lu form tetrahedral [RE(BH4)4] anionic complexes stabilized by Li+ cations crystallizing in the tetragonal space group P[4 with combining macron]2c. Additionally Sm and Gd also exhibit a transition to the LiRE(BH4)3Cl phases observed for the largest lanthanides. LiSm(BH4)3Cl is reduced to Sm(BH4)2 upon heating, which takes a previously unreported orthorhombic structure in space group Pbcn. Eu did not form any of these RE-borohydride compounds, but infrared spectroscopy indicates formation of a new borohydride phase. Hydrogen desorption from the composite mixtures starts around 200 °C. Rehydrogenation of the La-system under p(H2) = 10 MPa at 300 °C showed a partial reversibility, with 18% recovery of the original hydrogen capacity, which increased to 36% at 415 °C. Rehydrogenation under p(H2) = 10 MPa at 400 °C showed a partial reversibility of 25% in the Er-system. Formation of REH2 during thermal decomposition destabilizes the remaining LiBH4 in the composites, thus releasing hydrogen at lower temperatures than for pure LiBH4.

Graphical abstract: Structure and thermal properties of composites with RE-borohydrides (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Er, Yb or Lu) and LiBH4

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Article information


Submitted
30 Jul 2013
Accepted
23 Oct 2013
First published
31 Oct 2013

RSC Adv., 2014,4, 1570-1582
Article type
Paper

Structure and thermal properties of composites with RE-borohydrides (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Er, Yb or Lu) and LiBH4

J. E. Olsen, C. Frommen, T. R. Jensen, M. D. Riktor, M. H. Sørby and B. C. Hauback, RSC Adv., 2014, 4, 1570
DOI: 10.1039/C3RA44012E

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