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

Abstract

• Composites with very high hydrogen contents: NaBH 4 −2NH 3 BH 3 and KBH 4 −2NH 3 BH 3 . • Complementary techniques, in situ SR-PXD, 11 B 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 NaBH 4 –2NH 3 BH 3 and KBH 4 –2NH 3 BH 3 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 NaBH 4 or KBH 4 and NH 3 BH 3 do not react during ball milling or thermal treatment. Initially, ammonia borane decomposes independently of the presence of NaBH 4 or KBH 4 , as revealed by solid-state 11 B magic-angle spinning NMR, Fourier-transform infrared spectroscopy and powder X-ray diffraction. Samples of NaBH 4 –2NH 3 BH 3 and KBH 4 –2NH 3 BH 3 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 11 B isotropic chemical shift and quadrupole coupling parameters are reported for the boron site in polyiminoborane [NHBH] n .