Ab Initio Surface Models of Ruthenium Lithium Amide Catalytic Interfaces

Abstract

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.