Is the N2 dissociative mechanism universal across stepped metal surfaces for ammonia synthesis?

Abstract

The need of more efficient catalysts for thermal ammonia synthesis is urgent as ever, but is limited by the absence of innovative mechanisms beyond traditional dissociative mechanism. Recently, however, the associative mechanism has emerged as a viable route especially in electro-catalysis, calling for a deeper understanding in its possibility in catalyst design for thermal process. Here, we conduct a comprehensive analysis of elementary reactions on stepped metal surfaces, establishing scaling relationships for the associative mechanism and refining the ammonia synthesis volcano plot. Our results reveal that under typical reaction conditions (473–773 K, 1–100 bar), metal surfaces such as Ru, Fe, and Rh predominantly follow the dissociative mechanism with high reaction rates, while Mo, Ni, Pd, and Cu favor the associative mechanism but exhibit lower reaction rates. Expanding these finding through volcano plots to 21 metal surfaces, we observed that pure metal surfaces do not fall into the high-activity region for associative hydrogenation due to the scaling relationships. Therefore, we identify the N₂ direct hydrogenation step of N–HN bond formation as a key descriptor among the numerous elementary reaction steps of associative mechanism, and the efficiency of the associative pathway can be significantly enhanced by lowering the energy barrier of this step-particularly by breaking the linear scaling relationships. These findings challenge the traditional view of stepped metal surfaces favor dissociative mechanisms, and provide a new paradigm for catalysts design by reinventing reaction mechanisms.