Iron Nitride Formation and Decomposition during Ammonia Decomposition over a Wustite-Based Bulk Iron Catalyst

Hydrogen production using renewable sources shows great promise in lowering the dependence on fossil fuels in our current energy system, but challenges persist in its storage and transportation. Thus, ammonia emerges as a viable carrier for H2 due to its favorable properties. NH3 decomposition was studied at atmospheric pressure over a multiply promoted fused FeO-based bulk catalyst optimized for NH3 synthesis by applying transient kinetic experiments, in situ XRD, and Molecular dynamics (MD) simulations. Transient NH3 decomposition and temperature-programmed experiments in a fixed-bed reactor yielded the activation energies of bulk iron nitride formation and decomposition, which occurred in two steps: Fe ⇌ Fe4N ⇌ Fe2N. The decomposition of Fe4N into Fe and N2 was identified as the rate-determining step of NH3 decomposition with essentially the same activation energy amounting to 172 and 173 kJ mol-1, respectively. Fe4N and Fe2N were identified based on the nitrogen mass balance and confirmed as γ′-Fe4N and ϵ-Fe3N1.5 by in situ XRD experiments in NH3. MD simulations showed that the migration of adsorbed N atoms into the bulk of the catalyst is favored over recombinative desorption, influencing N storage and N2 release dynamics. Overall, the findings contribute to a comprehensive understanding of NH3 synthesis catalysts under decomposition conditions, identifying the key descriptor to enhance the performance of Fe-based catalysts.