Advances in catalytic ammonia synthesis over Fe- and Ru-based catalysts: Mechanisms and reaction kinetics

Abstract

Ammonia has recently received significant interest as a carbon-free fuel for the global energy transition, in addition to its use in various applications such as fertilizers, medicines, and polymers. This review provides an in-depth analysis of the thermal catalytic or Haber-Bosch (HB) ammonia synthesis process, including catalyst performance, as well as the associated challenges and future perspectives. Furthermore, the surface reaction mechanisms and kinetics of the nitrogen adsorption dissociation process during ammonia synthesis over iron (Fe) and ruthenium (Ru)-based catalysts are investigated. The rate equations, including those of Temkin & Pyzhev, Langmuir–Hinshelwood (LH), Ozaki et al., and the power rate law, are assessed and compared with experimental data obtained from Fe-based catalysts. The evaluation of the catalyst materials for the HB process demonstrated that the Co-based transition metal catalyst (Co/BaO/MgO) achieved a higher NH₃ synthesis rate than Ru and Fe-based catalysts. The results reveal that the rate expression proposed by Ozaki et al. exhibits a high degree of concordance with the experimental rate data, in contrast to the discrepancies observed with the Temkin rate equation. It is concluded that a comprehensive and detailed study of surface reaction mechanisms is essential to enhance the understanding of the ammonia synthesis process. Moreover, it is significant for the precise prediction of ammonia synthesis reaction rates and the optimization of catalytic performance.