Highly efficient ammonia synthesis at low temperature over a Ru-Co catalyst with dual atomically dispersed active...

The desire for a carbon-free society and the continuously increasing demand for clean energy make it valuable to exploit green ammonia (NH₃) synthesis that proceeds via the electrolysis driven Haber-Bosch (eHB) process. The key for successful operation is to develop advanced catalysts that can operate under mild conditions with efficacy. The main bottleneck of NH₃ synthesis under mild conditions is the known scaling relation in which the feasibility of N₂ dissociative adsorption of a catalyst is inversely related to that of the desorption of surface N-containing intermediate species, which leads to the dilemma that NH₃ synthesis could not be catalyzed effectively under mild conditions. The present work offers a new strategy via introducing atomically dispersed Ru onto a single Co atom coordinated with pyrrolic N, which forms RuCo dual single-atom active sites. In this system the d-band centers of Ru and Co were both regulated to decouple the scaling relation. Detailed experimental and theoretical investigations demonstrate that the d-bands of Ru and Co both become narrow, and there is a significant overlapping of t_2g and e_g orbitals as well as the formation of a nearly uniform Co 3d ligand field, making the electronic structure of the Co atom resemble that of a "free-atom". The "free-Co-atom" acts as a bridge to facilitate electron transfer from pyrrolic N to surface Ru single atoms, which enables the Ru atom to donate electrons to the antibonding π* orbitals of N₂, thus resulting in promoted N₂ adsorption and activation. Meanwhile, H₂ adsorbs dissociatively on the Co center to form a hydride, which can transfer to the Ru site to cause the hydrogenation of the activated N₂ to generate N₂H (x = 1-4) intermediates. The narrow d-band centers of this RuCo catalyst facilitate desorption of surface *NH₃ intermediates even at 50 °C. The cooperativity of the RuCo system decouples the sites for the activation of N₂ from those for the desorption of *NH₃ and *N₂H intermediates, giving rise to a favorable pathway for efficient NH₃ synthesis under mild conditions.