Antifluorite-Derived Li7MnN4: Revisiting the Crystal Structure and Catalysis in Ammonia Decomposition

Abstract

Catalytic ammonia decomposition is a sustainable chemical route for hydrogen production. Transition metal nitrides have emerged as promising and effective catalysts for this reaction. In this study, we revisit the synthesis, crystal structure, optoelectronic properties, and catalytic performance of antifluorite-derived Li7MnN4. Phase-pure Li7MnN4 powder is synthesized from Li3N and metallic Mn at 800°C in a tantalum ampoule, resulting in a highly crystalline cubic phase with space group P¯4 3n (No. 218), a lattice parameter of a = 9.5598(8) Å, and a unit cell volume of 873.66(14) Å3. Rietveld refinement results show excellent residual factors (Rwp = 1.71, S = 1.38), confirming the ordered arrangement of [MnN4]7– tetrahedra and five symmetrically distinct Li sites. The experimental data are complemented by density functional theory calculations, revealing weak spin coupling consistent with a paramagnetic ground state. Strong absorption in the UV-visible region corresponds to an experimental optical band gap of ~2.76 eV, while Raman and infrared spectra are dominated by MnN4 tetrahedral vibrations. X-ray absorption spectroscopy indicates a high Mn oxidation state and a well-defined Mn-N/Li coordination. Catalytic tests show that Li7MnN4 and Li7MnN4:LiNH2 exhibit activities comparable to a Ni-based reference catalyst, with apparent activation energies 364.4 kJ∙mol–1 and 256.0 kJ∙mol–1, respectively, highlighting the beneficial effect of LiNH2 incorporation. Thermogravimetry coupled with mass spectrometry identifies decomposition pathways involving LiNH2/Li2NH intermediates and forming Li3N and manganese nitrides. These results demonstrate that Li7MnN4 is a catalytically promising nitride for ammonia decomposition, with potential for further optimization through compositional tuning and mechanistic insights.