Thermal Decomposition of Anhydrous Alkali Metal Dodecaborates M2B12H12 (M = Li, Na, K)

Abstract

Metal dodecaborates M 2/n B 12 H 12 are regarded as the dehydrogenation intermediates of metal borohydrides M(BH 4 ) n that are expected to be high density hydrogen storage materials. In this work, thermal decomposition processes of anhydrous alkali metal dodecaborates M 2 B 12 H 12 (M = Li, Na, K) synthesized by sintering of MBH 4 (M = Li, Na, K) and B 10 H 14 have been systematically investigated in order to understand its role in the dehydrogenation of M(BH 4 ) n . Thermal decomposition of M 2 B 12 H 12 indicates multistep pathways accompanying the formation of H-deficient monomers M 2 B 12 H 12´x containing the icosahedral B 12 skeletons and is followed by the formation of (M 2 B 12 H z ) n polymers. The decomposition behaviors are different with the in situ formed M 2 B 12 H 12 during the dehydrogenation of metal borohydrides.

Properties and Applications of Metal (M) dodecahydro-closo-dodecaborates (Mn=1,2B12H12) and Their Implications for Reversible Hydrogen Storage in the Borohydrides

“…This polymerised phase was only observed to fully decompose to amorphous elemental boron when the temperature exceeded 650 • C [92]. This decomposition pathway has also been observed for NaB 12 H 12 , culminating in the formation of (Na 2 B 12 H z ) n polymers at around 700 • C [94].…”

“…The same unidentifiable intermediates were detected in Na 2 B 12 H 12 -10NaH samples treated under higher hydrogen pressure (400 • C under 97 MPa H 2 ). Although further investigation is required to clarify the composition of these intermediates, 11 B MAS NMR characterisation revealed structural similarities between the intermediates and [B 12 H 12 ] 2− , suggesting that they could be some form of closo-polyborate anion or polymerised icosahedral B 12 skeleton, as has been previously observed in studies of the thermal decomposition of NaB 12 H 12 [94].…”

“…From these observations, it was proposed that Li 2 B 12 H 12 decomposes via continuous hydrogen release, resulting in the formation of an amorphous hydrogen deficient dodecaborate species, Li 2 B 12 H 12−x [92,93]. After the formation of this hydrogen deficient phase, shifts of the major resonance peaks of [B 12 H 12 ] 2− indicate that the icosahedral B 12 skeleton of the anion may be polymerising, resulting in the formation of (Li 2 B 12 H z ) n polymers [94]. This polymerised phase was only observed to fully decompose to amorphous elemental boron when the temperature exceeded 650 • C [92].…”

Hydrogenation properties of lithium and sodium hydride – closo-borate, [B10H10]2− and [B12H12]2−, composites

“…The central-transition intensity for LiBH 4 is obtained as 4/10 of the total intensity, which holds for a spin 3/2 nucleus, and comparison of this value with the central transition intensity for 1 reveals that the molar ratio for boron in LiBH 4 and 1 is 1 : 0.63, which corresponds to an uptake of 2.8 wt% H 2 , matching well with the observed hydrogenation, 2.9 wt% H 2 . 44 This indicates that the absorbed hydrogen only goes toward producing LiBH 4 as the B : H ratio in the unknown compound 1 is 1 : 1, consistent with a higher borate, which is also reflected in its characteristic IR mode and 11 B chemical shift. Thus, 11 B MAS NMR reveals that all Li 2 B 10 H 10 (À0.9 ppm and À28.8 ppm) is consumed during hydrogenation and that LiBH 4 and 1 are the only reaction products, besides a minor amount of a BO 3 species (B20 ppm), corresponding to 3.5% of the boron in the sample.…”

Hydrides: Fundamentals and Applications

“…Our goal in assembling this special issue of Energies was to provide readers with a sense of the future directions that can be anticipated in metal hydride research in view of these changes. We hoped to accomplish this by providing a sampling of the variety of cutting-edge research efforts on metal hydrides that are currently underway throughout the scientific world.We are now happy to present the 15 outstanding contributions (13 original research papers and two reviews) that can be found within this special issue [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. With the inclusion of authors from 16 different countries, the issue truly presents a global view.…”