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 (VH) on the hydrides and (ii) reversible migration reaction of hydrogen adsorbed on Ru into the surface VH as H- (H + e- ↔ H-). Therefore, surface VH formation at the Ru/hydride interface is a key factor for ammonia synthesis with Ru/hydride catalysts. To promote VH formation at the Ru/hydride interface, we investigated the electronic structures of Ru-transition metal (TM) clusters loaded on a typical hydride, Ca2NH, and VH 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 VH 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.
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