First-Principles Study on the Mechanisms for H₂ Formation in Ammonia Borane at Ambient and High Pressure

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.

Positional disorder in ammonia borane at ambient conditions

“…We further used a 2 × 2 × 2 supercell, accommodating 16 molecules, and started from the experimental lattice constants 12 at 297 and 90 K for tetragonal and orthorhombic phases, respectively. Similar calculations have been done previously, 24 but with a different functional and at much higher temperature. Climbing image nudged-elastic band (NEB) simulations to find precise rotational barriers for halos and entire molecules were performed with Vasp (version 5.3.3), 25,26 utilizing PAW potentials and a cutoff of 500 eV.…”

Dynamics and fragmentation of van der Waals and hydrogen bonded cluster cations: (NH3)n and (NH3BH3)n ionized at 10.51 eV

“…To form a H 2 molecule from neutral NH 3 BH 3 on a reasonable, minimally energetic path, one H atom transfers from N to B through an energy barrier of 1.60 eV, and a H 2 molecule can be released from the BH 4 group. 21,22,27 A similar mechanism is explored for NH 3 BH 3 + ; however, the energy barrier is greater than 2.78 eV in the latter case. Moreover, the barrier for B-N bond breaking in NH 3 BH 3 is 1.13 eV, whereas it is 3.60 eV for NH 3 BH 3 + , which is 2.06 eV higher than the energy limit for the cation.…”

“…3 Ab initio calculations for the isolated NH 3 BH 3 molecule have provided important clues to the reaction pathways leading to the thermal decomposition of NH 3 BH 3 forming H 2 . In the gas phase, an H atom is transferred from the NH 3 moiety to boron of the same molecule and concomitantly the other two B-H bonds elongate, leading to the formation of H 2 with an activation energy of 32-33 kcal/mol: 21,22 this energy is significantly higher than that expected from the experimental decomposition temperature of about 80 • C. 27 The low temperature experimental rate constant maybe largely influenced by tunneling, 22 which can circumvent this large activation energy barrier. Thermogravimetric analysis of the solid suggests that the N-H• • •H-B and B-H• • •H-B fragmentation pathways for an H 2 product contribute nearly equal amounts of hydrogen below 120 • C, with the former becoming dominant after the solid has melted and molecules become mobile.…”

“…26 A dihydrogen bond is an analogue of the conventional hydrogen bond but involves the interaction between a positively charged (protic) hydrogen, bound to an atom of high electronegativity (N-H) and a hydrogen bonded to a less electronegative atom, a hydridic proton (B-H). 8,27,28 Formation of a dihydrogen network appears to be very important, as it leads to substantial stabilization of the single NH 3 BH 3 unit. 23 The H• • •H distance is around 1.7-2.2 Å, which is less than the sum of the van der Waals H atom radii of 2.4 Å: 2, 17,28 an average interaction energy of 3 kcal/mol (without zero-point correction) for the dihydrogen bond is predicted, 13,15,23 which shows that these bonds in ammonia borane crystals or clusters are relatively weak.…”

Lowering the hydrogen desorption temperature of NH3BH3 through B-group substitutions

“…One recent theoretical study 17 used enthalpy of formation calculations to conclude that intermolecular desorption pathways are more energetically favorable, but did not consider kinetic barriers. Another study 38 performed molecular dynamics simulations, observed an intramolecular desorption event, and found an energy barrier for the process, but did not consider intermolecular processes. A more recent experiment found that AB molecules polymerize in a "head-to-tail" manner through a dehydrocyclization reaction, 13 concluding that initial H 2 desorption occurs through intermolecular interactions.…”