Li₂ZrN₂: Crystal Structure, Electronic Properties, Oxidative Stability, Thermal Behavior, and Catalytic Activity in Ammonia Decomposition

Catalytic ammonia (NH_3(g)) decomposition is a carbon-neutral chemical process for hydrogen generation. Transition metal nitrides are particularly promising in this regard due to their unique catalytic properties. This study takes a closer look at the crystal structure, explores the thermal behavior under NH₃, and evaluates the catalytic activity of Li₂ZrN₂ for ammonia decomposition. Phase-pure Li₂ZrN₂, synthesized via solid-state reaction at 900 °C, crystallizes in the La₂O₃#CaAl₂Si₂-type structure in space group <i>P</i>3̅<i>m</i>1 (No. 164). Rietveld refinements and atomic parameters align well with previous studies. The unit cell parameters obtained are <i>a</i> = 3.2826(3) Å and <i>c</i> = 5.4611(5) Å. First-principles density-functional theory (DFT) calculations reveal an optical band gap of 2.50 eV, consistent with the experimentally determined value of 2.46 eV, and a low lattice thermal conductivity (1.52 W·m^-1·K^-1), suggesting its suitability for energy applications. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy confirm the ionic Li-N and covalent Zr-N bonding in Li₂ZrN₂. <i>In situ</i> X-ray diffraction analysis and thermogravimetric analysis coupled with mass spectrometry (TG-MS) reveal complex decomposition pathways of Li₂ZrN₂ under NH₃, impacting catalytic activity. Ammonia decomposition initiates above 500 °C and improves with successive heating-cooling cycles, likely due to the formation of active sites.