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

The desire for a carbon-free society and the continuously increasing demand for clean energy make it valuable to exploit green ammonia (NH) synthesis that proceeds 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 NH synthesis under mild conditions is the known scaling relation in which the feasibility of N 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 NH synthesis could not be catalyzed effectively under mild conditions. The present work offers a new strategy 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 and e 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 N, thus resulting in promoted N adsorption and activation. Meanwhile, H adsorbs dissociatively on the Co center to form a hydride, which can transfer to the Ru site to cause the hydrogenation of the activated N to generate NH ( = 1-4) intermediates. The narrow d-band centers of this RuCo catalyst facilitate desorption of surface *NH intermediates even at 50 °C. The cooperativity of the RuCo system decouples the sites for the activation of N from those for the desorption of *NH and *NH intermediates, giving rise to a favorable pathway for efficient NH synthesis under mild conditions.