Metal-free N₂-to-NH₃ thermal conversion at the boron-terminated zigzag edges of hexagonal boron nitride: Mechanism and kinetics

Nitrogen fixation is essential for all life and various industrial processes. Recent developments in the growth techniques of hexagonal boron nitride (hBN) enable controlled termination of hBN with zigzag edges. Here, we show that the B-terminated zigzag (B-ZZ) edge of hBN, which is hydrogenated and thus “defect-free”, can act as a metal-free catalyst for thermal conversion from N₂ and H₂ to NH₃ at high temperatures. Using density functional calculations, we identify the catalytic cycle of the NH₃ production, which involves simultaneous N₂ binding and hydrogenation at the one-dimensional edge of hBN. Further hydrogenation of the N₂-binding B-ZZ is facilitated by the H₂-induced local conversion between the sp² B and sp³ B sites at the B-ZZ edge. The NH₃ synthesis at the metal-free, defect-free B-ZZ edges, although less practical compared to the conventional Haber-Bosch process that uses transition metals, offers important insights into how the chemical flexibility of boron can be used for the challenging nitrogen transformations.