Investigations of the thermal decomposition of MBH4–2NH3BH3, M = Na, K

• Composites with very high hydrogen contents: NaBH4−2NH3BH3 and KBH4−2NH3BH3.
• Complementary techniques, in situ SR-PXD, 11B MAS NMR, TGA/DSC–MS and FTIR are used.
• No reactions occur during the mechanochemical or thermal treatment.
• Significant loss of boron as borazine and borane gasses during decomposition.

Thermal decomposition of NaBH4–2NH3BH3 and KBH4–2NH3BH3 prepared by ball milling are studied using in situ synchrotron radiation powder X-ray diffraction and simultaneous thermogravimetric analysis, differential scanning calorimetry and mass spectroscopy. The starting materials NaBH4 or KBH4 and NH3BH3 do not react during ball milling or thermal treatment. Initially, ammonia borane decomposes independently of the presence of NaBH4 or KBH4, as revealed by solid-state 11B magic-angle spinning NMR, Fourier-transform infrared spectroscopy and powder X-ray diffraction. Samples of NaBH4–2NH3BH3 and KBH4–2NH3BH3 were heated to 215 °C and subsequently investigated ex situ at RT. These samples consist of the respective metal borohydride and the decomposition product polyiminoborane. A significant loss of borane and borazine gasses from the samples is observed during decomposition. Therefore, the present samples do not store hydrogen reversibly at the conditions used in this study. Finally, the 11B isotropic chemical shift and quadrupole coupling parameters are reported for the boron site in polyiminoborane [NHBH]n.