In situ solid state 11B MAS-NMR studies of the thermal decomposition of ammonia borane: mechanistic studies of the hydrogen release pathways from a solid state hydrogen storage material

Abstract

The mechanism of hydrogen release from solid state ammonia borane (AB) has been investigated via in situ solid state (11)B and (11)B{(1)H} MAS-NMR techniques in external fields of 7.1 T and 18.8 T at a decomposition temperature of 88 degrees C, well below the reported melting point. The decomposition of AB is well described by an induction, nucleation and growth mechanistic pathway. During the induction period, little hydrogen is released from AB; however, a new species identified as a mobile phase of AB is observed in the (11)B NMR spectra. Subsequent to induction, at reaction times when hydrogen is initially being released, three additional species are observed: the diammoniate of diborane (DADB), [(NH(3))(2)BH(2)](+)[BH(4)](-), and two BH(2)N(2) species believed to be the linear (NH(3)BH(2)NH(2)BH(3)) and cyclic dimer (NH(2)BH(2))(2) of aminoborane. At longer reaction times the sharper features are replaced by broad, structureless peaks of a complex polymeric aminoborane (PAB) containing both BH(2)N(2) and BHN(3) species. The following mechanistic model for the induction, nucleation and growth for AB decomposition leading to formation of hydrogen is proposed: (i) an induction period that yields a mobile phase of AB caused by disruption of the dihydrogen bonds; (ii) nucleation that yields reactive DADB from the mobile AB; and (iii) growth that includes a bimolecular reaction between DADB and AB to release the stored hydrogen.