Beyond transition metals: rare-earth phthalocyanines as single-atom catalysts for efficient nitrogen reduction

The development of efficient electrocatalysts for ambient ammonia synthesis is a key challenge for enabling sustainable nitrogen fixation. However, most reported single-atom catalysts (SACs) rely heavily on conventional transition metals, which limits the exploration of alternative active centers. In this work, first-principles calculations are employed to systematically investigate a series of rare-earth (RE) SACs, focusing on their stability, nitrogen reduction reaction (NRR) activity, reaction pathways, and selectivity. The results indicate that the alternating pathway is thermodynamically preferred for the NRR, and a volcano-type relationship is established between the limiting potential and the adsorption free energy of NNH*, identifying NNH* as an effective activity descriptor. Among the investigated systems, Ce-embedded phthalocyanine (Ce/PC) exhibits excellent stability, high NRR activity, and strong suppression of the competing hydrogen evolution reaction. Electronic structure analysis reveals that the interaction between Ce and the PC support is mainly governed by Ce-5d orbitals, while activation of the N₂ π* antibonding orbital originates from charge transfer involving Ce-4f states. Moreover, axial Li coordination further enhances the catalytic performance of Ce/PC, reducing the limiting potential to -0.13 V. Overall, this study broadens the scope of SAC design beyond traditional transition metals and provides new insights into rare-earth-based catalysts for sustainable ammonia synthesis.