A spin-crossover-mediated potential-dependent selective NO reduction reaction on iron-polyphthalocyanine: a DFT study

The electrochemical reduction of nitric oxide (NORR) represents a promising route for both mitigating nitrogen oxide pollution and enabling sustainable ammonia synthesis. Iron-polyphthalocyanine (FePPc), characterized by its well-defined Fe-N₄ structure, has shown remarkable catalytic activity; however, its spin-involved reaction mechanism and its impact on selectivity remain unclear. Moreover, the influence of spin-dependent effects on the reaction pathways in graphene-based single-atom catalysts has received scant attention, limiting the fundamental understanding of spin-activity relationships in electrocatalysis. In this work, we employ constant-potential density functional theory (DFT) calculations to investigate the NORR mechanism on FePPc, focusing on the competition between NH₃ and NH₂OH formation. Our results reveal that the spin state of key intermediates is potential-dependent, with spin crossover occurring at critical reaction steps. This potential-driven spin-crossing phenomenon directly dictates the reaction trajectory, leading to a shift in product selectivity between NH₃ and NH₂OH under varying applied potentials. We demonstrate that the spin-crossover mechanism is essential for understanding and controlling product distribution under operational electrochemical conditions. These findings provide new insights into the role of spin states in tuning the selectivity of single-atom catalysts for nitrogen oxide reduction and highlight spin engineering as a critical design principle for advanced electrocatalysts.