ARTICLE pubs.acs.org/JPCC
First-Principles Study on the Mechanisms for H2 Formation in Ammonia Borane at Ambient and High Pressure Yunfeng Liang†,‡ and John S. Tse*,† † ‡
Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon S7N5E2, Canada Department of Urban Management, Kyoto University, Kyoto 615-8540, Japan ABSTRACT: First-principles molecular dynamics calculations were performed to investigate the mechanism on thermal decomposition of H2 of ammonia borane (NH3BH3) at ambient and high pressure. Under atmospheric pressure, one H2 molecule was released through an intramolecular reaction of a single NH3BH3. Nudged elastic band calculations show that the activation barrier for the decomposition in the crystalline environment is reduced by almost 1/3 from the gas phase value. When the system is heated under pressure two H2 were liberated. The reaction follows an intermolecular pathway involving the concerted interaction of three NH3BH3. The results demonstrate the importance and practical significance of pressure on increasing the evolution of H2 from NH3BH3.
1. INTRODUCTION Hydrogen-based fuel cell is an environmentally friendly energy carrier for transportation applications. Ammonia borane (AB) is a stable lightweight solid1 6 containing a high density of hydrogen that can be thermally liberated in the temperature range 350 410 K for use in fuel cell powered applications.7 9 Very recently, it has been demonstrated that AB fuel waste could be converted back to AB in a sealed pressure vessel with a N2H4/NH3 feed.10 The high hydrogen content, the moderate decomposition temperature, the exothermic nature of the decomposition process and the breakthrough of the bottleneck for the regeneration of AB allow interesting applications of AB as a hydrogen source for fuel cells. Experimentally, it was shown that using nanoporous silica as scaffold, CO2 as catalyst, or doping samples, the kinetics of hydrogen release can be enhanced and the exothermic nature of the reaction was suppressed.11 16 Despite the significant interest of this material, the mechanism for hydrogen release is not fully understood. Reliable information on the decomposition mechanism17 20 will help to select the optimal atmosphere and tailor the doping process for future development. Although ethane and AB are isoelectronic, there are significant differences in their physical properties. The most notable is the melting point where under ambient conditions C2H6 is a gas with a melting point of 181 °C as compared with that of AB at 104 °C. The discrepancy has been attributed to the presence of a “di-hydrogen bond” N Hδ+ 3 3 3 δ‑H B between the protonic hydrogen from NH3 group and hydridic hydrogen from BH3 group of an adjacent molecule. This interaction is absent in ethane. The dihydrogen bond plays a significant role in determining the properties of the solid AB4 and thermally5,6 and pressure-induced21 29 phases. At ambient pressure and low temperature (
