Enhanced Ammonia Synthesis via Sm Doping in Ru/La₂Ce₂O₇ Catalysts: Insights into Structural Analysis and Oxygen Vacancy

Ammonia (NH₃) has great potential as a hydrogen carrier material and plays an important role in chemistry. NH₃ is synthesized through a conventional Haber-Bosch process; however, this method requires considerable energy. Thus, it is important to develop NH₃ synthesis catalysts that operate under mild conditions. This research revealed that Sm-doped Ru/La₂Ce₂O₇ catalysts (Ru/La₂Ce_2-<i>x</i>Sm_<i>x</i>O₇) increased the NH₃ synthesis under mild conditions (400 °C, 0.1 MPa). Catalyst with an optimal Sm doping ratio (<i>x</i> = 0.75) showed the highest NH₃ productivity of 4563 μmol h^-1 g^-1. This result is mainly attributed to the transition from a fluorite to a C-type structure by introducing Sm to Ru/La₂Ce₂O₇, since the C-type structure possesses a higher abundance of oxygen vacancies compared to fluorite. This structural transition facilitated electron transfer to the active Ru metal and enhanced hydrogen spillover on the support, substantially improving the performance of the optimized catalysts. Nevertheless, the introduction of Sm dopants brought certain deactivation effects, including blocking active sites on Ru and promoting a high degree of ordering in the oxygen vacancy, which induced the degradation of electron mobility. As a result, it became evident that not just the concentration but also the arrangement of oxygen vacancies critically influences the catalytic performance. These results underscore the importance of precisely tuning the Sm doping level to achieve a balance between its beneficial and adverse effects on the structural and electronic properties, which is critical for maximizing ammonia synthesis activity.