Origin of Enhanced Ammonia Synthesis on Ru-Co Catalysts Unraveled by Density Functional Theory

Bimetallic Ru-Co catalysts have been reported as promising NH3 production catalysts that are better than various Ru-X alloys, including RuFe and pure Ru, under mild conditions; however, a systematic understanding of their superior activity is still lacking. Here, we report a comprehensive theoretical study on NH3 synthesis of Ru-Co catalysts using density functional theory and microkinetic modeling. Indeed, the RuCo surface enables more facile N-2 dissociation than the Ru surface, which results from the manifested Co-induced spin symmetry breaking of Ru. We also investigated the surface phase diagram of RuCo(0001) with partial pressures of H-2 and N-2 gases and found that the most stable phase of the RuCo surface consists of a fraction of both N and H atoms under experimental Haber-Bosch pressure conditions; however, NH3 can be readily produced on the surfaces without severe surface poisoning issues. Furthermore, this study shows that the spin-symmetry breaking of nonmagnetic surfaces can enhance the catalytic activity for NH3 synthesis, which provides an alternative strategy to catalyst design.