Revisiting Ordered Antifluorite-Type Li14Cr2N8O: Synthesis, Crystal Structure, Theoretical Perspectives, and Catalytic Activity for Ammonia Decomposition

Abstract

Ammonia is an efficient compound for hydrogen transportation. The release of hydrogen from ammonia at the point of use is accomplished by the catalytic decomposition of ammonia using commercially available catalysts. Transition metal nitrides with properties similar to those of well-known precious metal-based catalysts exhibit outstanding catalytic activity in ammonia decomposition. Particularly, the cyclical formation and decomposition of certain lithium-based ternary metal nitrides result in improved catalytic activity in the decomposition of ammonia. Since the stability of lithium metal nitride oxides generally exceeds that of lithium metal nitrides, catalysts based on lithium metal nitride oxides are of particular interest for future practical applications. Therefore, this study aims at revisiting the synthesis, crystal structure, and stability from the theoretical perspective of Li₁₄Cr₂N₈O, as a member of the lithium-based transition metal nitride oxide family. A one-step method is applied to successfully synthesize single-phase Li₁₄Cr₂N₈O in powder form with high crystallinity. The crystal structure of Li₁₄Cr₂N₈O is determined to adopt the Na₁₄Mn₂O₉-type structure. The group-subgroup relation between the antifluorite-type structure and Li₁₄Cr₂N₈O with the Na₁₄Mn₂O₉-type structure is elucidated by applying the Bärnighausen formalism. Rietveld refinements and atomic parameters from the literature show equally satisfactory results. The unit-cell parameters obtained for Li₁₄Cr₂N₈O are a = 5.7936(6) Å and c = 8.20634(11) Å (trigonal crystal system). Theoretical studies by density functional theory (DFT) are conducted to explore the stability of Li₁₄Cr₂N₈O with respect to anion order and exchange, the magnetic ground state, and the oxidation state of chromium. The findings of this study pave the way for Li₁₄Cr₂N₈O to be further explored as an important catalyst for the decomposition of ammonia to generate hydrogen.