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 N-2 and H-2 to NH3 at high temperatures. Using density functional calculations, we identify the catalytic cycle of the NH3 production, which involves simultaneous N-2 binding and hydrogenation at the one-dimensional edge of hBN. Further hydrogenation of the N-2-binding B-ZZ is facilitated by the H-2-induced local conversion between the sp(2)B and sp(3)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. (C) 2019 Elsevier Inc. All rights reserved.