Microscopic Mechanism of Enhanced Catalytic Activity for Ammonia Synthesis in Y5M3 (M = Si/Ge) Electrides

Due to the importance of ammonia in modern agriculture, industry, scientific research, and so on, exploring a high-performance catalyst for ammonia synthesis at mild conditions is always highly desirable and will continuously be a hot research area. In recent years, electride catalysts have drawn considerable attention due to the presence of excess interstitial electrons. Among the known electride catalysts, the electride Y5Si3 is particularly interesting owing to its excellent catalytic activity in both air and water conditions. However, the underlying microscopic mechanism for the whole reaction process of the ammonia synthesis on the Y5Si3 surface remains unclear, in particular for the role played by the interstitial electrons. Here, through first principles calculations, we have systematically studied the adsorption structures and energies of an atom (H and N) or a molecule (H2, N2, and NH3), the dissociation of H2 and N2 molecules, and the energy barriers for the formation of intermediate complexes such as NH and NH2 and the final NH3 on the Y5Si3(0001) surface. Our charge density analysis unambiguously demonstrates that the surface interstitial electrons play a very important role in facilitating the N2 dissociation as well as the formation of NH3 by providing necessary electrons that reduce the energy barriers. Moreover, we have carried out a comparative study on another isostructural and isoelectronic electride Y5Ge3. We find that the energy barriers in the rate-determining step on both electride surfaces are lower than that on Ru(0001), suggesting their good catalytic activities in synthesizing NH3. We further show that the conclusion remains unchanged by the presence of a N atom occupying the sublayer octahedral interstitial site.