Revealing the potential-determining steps of reduction of nitrate to ammonia on transition metal porphyrins catalysts

The reduction reaction of nitrate (NO₃RR) to ammonia by electrocatalytic methods is a promising method for sustainable ammonia synthesis, yet it remains a great challenge. In this work, we report a class of transition metal coordination porphyrins (TM-PP) as a cost-effective, high activity and selectivity electrocatalysts for NO₃RR. By large-scale density functional theory calculations, our investigation into the catalytic mechanism of TM-PP sheds light on the underlying principles of their enhanced activity. It's worth noting that through systematic study of potential-determining steps, we proposed for the first time that the Gibbs free energy change of the 7th step (*NOH→*N) reaction can serve as an effective catalytic descriptor for NO₃RR and unveiled its underlying mechanism. We theoretically demonstrate that Mo-PP and Ru-PP stand out among 23 TM-PP and showcasing superior performance supported by systematic analyses of kinetics, thermodynamics, and electrochemical stabilities. Mo-PP boast a significantly reduced additional voltage requirement of merely –0.16 V compared to existing catalysts. Furthermore, both Mo-PP and Ru-PP demonstrate the capacity to suppress the competing HER and effectively prevent the generation of by-products. Overall, our work presents a promising NO₃RR catalyst with great potential for practical applications and provides valuable insights into the NO₃RR intricate mechanisms.