J. Phys. Chem. C 2010, 114, 19529–19534
19529
Thermal Decomposition and Spectroscopic Studies of Preheated Ammonia Borane Junshe Zhang,† Yu Zhao,‡ Daniel L. Akins,‡ and Jae W. Lee*,† Department of Chemical Engineering and Department of Chemistry, The City College of New York, New York, New York 10031, United States ReceiVed: June 1, 2010; ReVised Manuscript ReceiVed: September 27, 2010
This paper presents enhanced dehydrogenation of solid ammonia borane (AB, NH3BH3) at 85 °C after it was preheated at 80 °C for 2-4 h. Thermal treatments and subsequent hydrogen release experiments were carried out in a Hastelloy cell mounted on a high-pressure differential scanning calorimeter. With increased preheating durations, not only does the induction period for hydrogen release shorten but also the hydrogen release becomes fast. It is deduced that diammoniate of diborane (DADB, [(NH3)2BH2]+[BH4]-), oligomers or polymers, and AB form a solid solution, shifting the melting temperature of AB to a low value. Hydrogen evolution, as the result of the initial stage of oligomerization, is observed after maintaining pristine AB at 80 °C for 20 min. FTIR-ATR spectra of thermally treated AB indicated that only linear dimers of aminoborane (LDAB, NH3BH2NH2BH3) are detected for preheating periods shorter than 3 h, whereas other oligomers and polymers form for the longer thermal treatment at 80 °C. 1. Introduction Ammonium borane (AB, NH3BH3) is being considered as a promising on-board hydrogen storage medium due to its high storage capacity (19.6 wt %) and moderate dehydrogenation temperatures. The thermal decomposition of solid AB includes three consecutive steps, occurring at around 110, 150, and >500 °C, with about 6.5 wt % (with respect to the mass of AB) hydrogen released in each step.1 However, utilization of AB as a chemical hydrogen storage material for mobile applications is impeded by the formation of borazine and unfavorable dehydrogenation kinetics, for example, long induction period and low hydrogen release rate at low temperatures (
