Hydrogen-fluorine exchange in NaBH sub(4)-NaBF sub(4)

Hydrogen-fluorine exchange in the NaBH sub(4)-NaBF sub(4) system is investigated using a range of experimental methods combined with DFT calculations and a possible mechanism for the reactions is proposed. Fluorine substitution is observed using in situsynchrotron radiation powder X-ray diffraction...

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Published in:Physical chemistry chemical physics : PCCP 2013-10, Vol.15 (41), p.18185-18194
Main Authors: Rude, L H, Filsoe, U, D'Anna, V, Spyratou, A, Richter, B, Hino, S, Zavorotynska, O, Baricco, M, Soerby, M H, Hauback, B C, Hagemann, H, Besenbacher, F, Skibsted, J, Jensen, T R
Format: Article
Language:English
Subjects:
Decomposition
Fluorine
Foaming
Hydrogen storage
Mathematical analysis
Preserves
Uptakes
X-rays
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Summary:Hydrogen-fluorine exchange in the NaBH sub(4)-NaBF sub(4) system is investigated using a range of experimental methods combined with DFT calculations and a possible mechanism for the reactions is proposed. Fluorine substitution is observed using in situsynchrotron radiation powder X-ray diffraction (SR-PXD) as a new Rock salt type compound with idealized composition NaBF sub(2)H sub(2) in the temperature range T= 200 to 215 degree C. Combined use of solid-state super(19)F MAS NMR, FT-IR and DFT calculations supports the formation of a BF sub(2)H sub(2) super(-) complex ion, reproducing the observation of a super(19)F chemical shift at -144.2 ppm, which is different from that of NaBF sub(4) at -159.2 ppm, along with the new absorption bands observed in the IR spectra. After further heating, the fluorine substituted compound becomes X-ray amorphous and decomposes to NaF at similar to 310 degree C. This work shows that fluorine-substituted borohydrides tend to decompose to more stable compounds, e.g. NaF and BF sub(3) or amorphous products such as closo-boranes, e.g. Na sub(2)B sub(12)H sub(12). The NaBH sub(4)-NaBF sub(4) composite decomposes at lower temperatures (300 degree C) compared to NaBH sub(4) (476 degree C), as observed by thermogravimetric analysis. NaBH sub(4)-NaBF sub(4) (1 : 0.5) preserves 30% of the hydrogen storage capacity after three hydrogen release and uptake cycles compared to 8% for NaBH sub(4) as measured using Sievert's method under identical conditions, but more than 50% using prolonged hydrogen absorption time. The reversible hydrogen storage capacity tends to decrease possibly due to the formation of NaF and Na sub(2)B sub(12)H sub(12). On the other hand, the additive sodium fluoride appears to facilitate hydrogen uptake, prevent foaming, phase segregation and loss of material from the sample container for samples of NaBH sub(4)-NaF.
ISSN:1463-9076
1463-9084
DOI:10.1039/c3cp52815d