Promoting N2 Reactivity by Floating Electrons on Two-Dimensional Electride Surfaces

Abstract The activation of N2 is a critical step in ammonia synthesis but remains challenging because of the extremely high bonding energy of the N2 molecule. The present work demonstrates that two-dimensional (2D) layered electrides having the general formula Ae2N (Ae = Ca, Sr, Ba) act as electron donors to promote N2 activation. Analyses based on the electron localization function show that floating electrons on the (104) surface of Ae2N compounds directly interact with the empty π* orbitals of N2 molecules, thus promoting chemisorption at van der Waals (vdW) interstitial sites. Constrained ab initio molecular dynamics simulations indicate that the energy barrier to N2 dissociation on Ca2N and Ba2N is similar. However, exposure to a nitrogen atmosphere converts Ca2N to β-Ca3N2, which explains the experimentally observed inactivity of Ca2N toward N2 by isotopic exchange reaction. In contrast to the fast trapping of nitrogen ions in the vdW gap in Ca2N, nitrogen ions in Ba2N exhibit rapid surface migration that may facilitate N2 dissociation and recombination. During the exchange of N2 with lattice nitrogen species, the newly formed N2 is released directly from the Ba2N surface, whereas stronger Ae-N interactions on Ca2N and Sr2N provide an energy barrier to N2 release. These results identify the key factors governing efficient N2 activation on 2D electrides, providing insights into transition-metal-free N2 activation under mild conditions.