New hydrogen-rich ammonium metal borohydrides, NH4[M(BH4)(4)], M = Y, Sc, Al a potential H-2 sources

摘要

Three metal-ammonium borohydrides, NH4[M(BH4)(4)] M = Y, Sc, Al, denoted 1, 2, 3, respectively, were prepared via a low temperature mechanochemical synthesis and characterized using PXRD, FTIR and TGA/DSC/MS. The compounds 1 and 2 adopt the P2(1)/c space group while the compound 3 crystallizes in an orthorhombic unit cell (Facia). The first decomposition step of all three derivatives of ammonium borohydride has the maximum rate at 48 degrees C, 53 degrees C and 35 degrees C for 1, 2 and 3, respectively, which are comparable to that for NH4BH4 (53 degrees C). The thermal decomposition of these metal-ammonium borohydrides is a multi-step process, with predominantly exothermic low-temperature stages. The compound 1 decomposes via known Y(BH4)(3), however, some of the solid decomposition products of the other two compounds have not been fully identified. In the system containing compound 2, a new, more dense polymorph of the previously reported LiSc(BH4)(4) has been detected as the intermediate of slow decomposition at room temperature.