Promoting N ₂ Reactivity by Floating Electrons on Two-Dimensional Electride Surfaces

High Resolution Image Download MS PowerPoint Slide The activation of N 2 is a critical step in ammonia synthesis but remains challenging because of the extremely high bonding energy of the N 2 molecule. The present work demonstrates that two-dimensional (2D) layered electrides having the general formula Ae 2 N ( Ae = Ca, Sr, Ba) act as electron donors to promote N 2 activation. Analyses based on the electron localization function show that floating electrons on the (104) surface of Ae 2 N compounds directly interact with the empty π* orbitals of N 2 molecules, thus promoting chemisorption at van der Waals (vdW) interstitial sites. Constrained ab initio molecular dynamics simulations indicate that the energy barrier to N 2 dissociation on Ca 2 N and Ba 2 N is similar. However, exposure to a nitrogen atmosphere converts Ca 2 N to β-Ca 3 N 2, which explains the experimentally observed inactivity of Ca 2 N toward N 2 by isotopic exchange reaction. In contrast to the fast trapping of nitrogen ions in the vdW gap in Ca 2 N, nitrogen ions in Ba 2 N exhibit rapid surface migration that may facilitate N 2 dissociation and recombination. During the exchange of N 2 with lattice nitrogen species, the newly formed N 2 is released directly from the Ba 2 N surface, whereas stronger Ae -N interactions on Ca 2 N and Sr 2 N provide an energy barrier to N 2 release. These results identify the key factors governing efficient N 2 activation on 2D electrides, providing insights into transition-metal-free N 2 activation under mild conditions.