Electronic Structure and Initial Dehydrogenation Mechanism of M(BH₄)₂·2NH₃ (M = Mg, Ca, and Zn): A First-Principles Investigation

Abstract

The electronic structure and initial dehydrogenation mechanism of M­(BH₄)₂·2NH₃ (M = Mg, Ca, and Zn) have been systematically studied using first-principles calculations. A detailed study of the electronic structure reveals that the metal cations in M­(BH₄)₂·2NH₃ play a crucial role in both suppressing ammonia emission and destabilizing the N–H/B–H bonds. The calculation results of hydrogen removal energies are in agreement with the tendency of dehydrogenation temperatures of these ammoniates, i.e., Zn­(BH₄)₂·2NH₃ < Mg­(BH₄)₂·2NH₃ < Ca­(BH₄)₂·2NH₃. The initial dehydrogenation of M­(BH₄)₂·2NH₃ is achieved by the dissociation of (N)­H^δ+ from NH₃ and (B)­H^δ− atoms from BH₄ groups, resulting in the formation of N–B dative bonds and the reduction of the neighboring (N)­H^δ+···(B)­H^δ− dihydrogen bonds, which accelerate the subsequent dehydrogenation