Ab Initio Surface Models of Ruthenium Lithium Amide Catalytic Interfaces

Density functional theory calculations are performed to develop atomistic surface models of ruthenium lithium amide catalytic interfaces. In preamble, the stability study of all the LiNH₂(001) terminations demonstrates a preference for NH₂-terminated surfaces with hydrogen pointing toward vacuum and a possibility to reconstruct metastable systems by rotating all the terminal NH₂ moieties. Then, the elaboration of interface models between Ru(0001) and LiNH₂(001) surfaces is performed, which is strongly inspired from a preliminary study of isolated adsorption of lithium and NH₂ on Ru(0001). The competitive adsorption of these species leads to two possible contacts to interface Ru and LiNH₂ materials: either through lithium or NH₂. Numerous interface models are optimized by considering the influences of LiNH₂(100) thickness, its stoichiometry, and its polarity, including spin polarization and van der Waals interactions. The most stable interfaces are composed of NH₂ planes chemisorbed on Ru(0001). The contact via Li atoms leads to a family of highly metastable interfaces. Surprisingly, the five most competitive interfaces are all non-stoichiometric. The interface stability is then correlated to Bader charge transfer analysis. This study questions the specific catalytic role of lithium in the context of hydrogen storage and the reaction mechanism of ammonia decomposition on ruthenium lithium amide catalysts.