Transition Metal-doped Ru Nanoparticles Loaded on Metal Hydrides for Efficient Ammonia Synthesis from First Principles

Ru-loaded hydrides work as efficient catalysts for ammonia synthesis at low temperatures. The advantages of high activity when using Ru-loaded hydride catalysts are as follows: (i) electron injection from the hydrides to Ru, mediated by surface hydrogen vacancies (V_H) on the hydrides and (ii) reversible migration reaction of hydrogen adsorbed on Ru into the surface V_H as H^- (H + e^- ↔ H^-). Therefore, surface V_H formation at the Ru/hydride interface is a key factor for ammonia synthesis with Ru/hydride catalysts. To promote V_H formation at the Ru/hydride interface, we investigated the electronic structures of Ru-transition metal (TM) clusters loaded on a typical hydride, Ca₂NH, and V_H formation and H-migration reaction at the Ru-TM/Ca₂NH interface using the Ru₅TM/Ca₂NH model with density functional theory calculations. Five late TMs (Fe, Co, Rh, Os, and Ir) and eight early TMs (Sc, Ti, Y, Zr, Nb, La, Hf, and Ta) were determined to promote V_H formation. Nb, Hf, Ta, Os, and Ir can also decrease the H-migration energy at the Ru₅TM/Ca₂NH interface when compared with that at the Ru₆/Ca₂NH interface.