Iron Nitride Formation and Decomposition during Ammonia Decomposition over a Wustite-Based Bulk Iron Catalyst
Maximilian Purcel, Stefan Berendts, Luigi Bonati, Simone Perego, A. Müller, Martin Lerch, Michele Parrinello, Martin Muhler
Abstract
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 H 2 due to its favorable properties. NH 3 decomposition was studied at atmospheric pressure over a multiply promoted fused FeO-based bulk catalyst optimized for NH 3 synthesis by applying transient kinetic experiments, in situ XRD, and Molecular dynamics (MD) simulations. Transient NH 3 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 ⇌ Fe 4 N ⇌ Fe 2 N. The decomposition of Fe 4 N into Fe and N 2 was identified as the rate-determining step of NH 3 decomposition with essentially the same activation energy amounting to 172 and 173 kJ mol –1, respectively. Fe 4 N and Fe 2 N were identified based on the nitrogen mass balance and confirmed as γ′-Fe 4 N and ε-Fe 3 N 1.5 by in situ XRD experiments in NH 3 . 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 N 2 release dynamics. Overall, the findings contribute to a comprehensive understanding of NH 3 synthesis catalysts under decomposition conditions, identifying the key descriptor to enhance the performance of Fe-based catalysts.