Ammonia Synthesis on Ternary LaSi-based Electrides: Tuning the Catalytic Mechanism by the Third Metal

Intermetallic electrides have recently drawn considerable attention due to their unique electronic structure and high catalytic performance for the activation of inert chemical bonds under mild conditions. However, the relationship between electride (anionic) electron abundance and catalytic performance is undefined; the key deciding factor for the performance of intermetallic electride catalysts remains to be addressed. Here, the secret behind electride catalysts La-TM-Si (TM=Co, Fe and Mn) with the same crystal structure but different anionic electrons was studied. Unexpectedly, LaCoSi with the least anionic electrons showed the best catalytic activity. The experiments and first-principles calculations showed that the electride anions promote the N₂ dissociation which alters the rate-determining step (RDS) for ammonia synthesis on the studied electrides. Different reaction mechanisms were found for La-TM-Si (TM=Fe, Co) and LaMnSi. A dual-site module was revealed for LaCoSi and LaFeSi, in which transition metals were available for the N₂ dissociation and La accelerates the NH_x formation, respectively, breaking the Sabatier scaling relation. For LaMnSi, which is the most efficient for the N₂ activation, the activity for ammonia synthesis is limited and confined by the scaling relations. The findings provide new insight into the working mechanism of intermetallic electrides.