(532ac) Computational Screening and Experimental Validation of Binary and Ternary Metal Nitrides for the Solar-Driven Thermochemical Production of Green Ammonia

The current conventional production of ammonia relies on the well-known Haber-Bosch process, involving a catalytic high-pressure reaction between H₂ and N₂. Due to the production of H₂ and N₂ based on highly energy-intensive processes using fossil fuels, the worldwide ammonia production is responsible for 1.2% of the anthropogenic global greenhouse gas emissions. Furthermore, the high pressures needed to increase the yield and the large recycle flows of the unreacted H₂ and N₂ impose demanding requirements on the equipment, increasing the cost and complexity of the process and favouring large, centralized plants. Multi-step thermochemical cycles based on metal nitrides stand as a promising alternative to this process, since they can substantially mitigate or even eliminate the concomitant CO₂ emissions linked to ammonia production. In such cycles, concentrated solar energy is used to supply the high-temperature heat required in the endothermic reaction steps. Previous studies have proven successful synthesis of ammonia at much lower pressures – even around ambient conditions. Nevertheless, the availability of literature and experimental data on the metal nitrides involved in these cycles is scarce. In an effort to investigate a broader range of candidates, this works presents the results of a screening of different metal nitride compounds using DFT (Density Function Theory) calculations from open-access databases. The probable reaction pathways encompassing either the hydrogenation (H₂) or the hydrolysis (H₂O) of such nitrides, as well as their re-nitridation to recycle the pristine metal nitride were identified through a Gibbs free energy minimization algorithm. The experimental validation of the selected candidates was conducted both through dynamic thermogravimetric analysis (TGA) and in a high-pressure reactor. Finally, the different fresh and spent materials were submitted to physicochemical characterization, to evaluate the chemical, structural and morphological changes inferred through hydrogenation/hydrolysis/re-nitridation and aiming to assess their performance under cyclic operation.