PNNL

Abstract

The mechanism of hydrogen release from solid state ammonia borane (AB) has been investigated via in situ solid state 11B{1H} MAS-NMR techniques in external fields of 7.06 T and 18.8 T at a decomposition temperature of 88 oC, 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 11B NMR spectra. Subsequent to induction, at reaction times when hydrogen is initially being released, three additional species are observed: the diammoniate of diborane (DADB), [(NH3)2BH2]+[BH4]-, and two BH2N2 species believed to be the linear (NH3BH2NH2BH3) and cyclic dimer (NH2BH2)2 of aminoborane. At longer reaction times the sharper features are replaced by broad, structureless peaks of a complex polymeric aminoborane (PAB) containing both BH2N2 and BHN3 species. We propose the following mechanistic model for the induction, nucleation and growth for AB decomposition leading to formation of hydrogen: (1) an induction period that yields a mobile phase of AB caused by disruption of the dihydrogen bonds, (2) nucleation that yields reactive DADB from the mobile AB and (3) growth that includes a bimolecular reaction between DADB and AB to release the stored hydrogen. Support for this work by the U.S. Department of Energy, Office of Science, Basic Energy Sciences is gratefully acknowledged. A portion of the research described in this paper was performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the U.S. Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.