Elucidating the Impact of the Metal Center Identity and Macrocycle Structure of Molecular Catalysts for Electrochemical Reduction of Nitrate to Ammonia

Ammonia (NH3) is a vital fertilizer and industrial chemical predominantly produced via the energy-intensive Haber–Bosch process. The electrochemical reduction of nitrate (NO3–) to NH3 offers an alternative that can source nitrogen from NO3– in wastewater or industrial processes. In this work, we evaluate the impact of the local atomic environment of four molecular catalysts supported on carbon nanotubes (CNTs): copper phthalocyanine (CuPc/CNT), copper tetraphenylporphyrin (CuTPP/CNT), iron phthalocyanine (FePc/CNT), and iron tetraphenylporphyrin (FeTPP/CNT). FePc/CNT coated electrodes achieved the highest performance, exhibiting a partial current density of 61.2 mA cm–2 at −0.9 VRHE and a Faradaic efficiency of 98.9% toward NH3 at −0.6 VRHE. Notably, the phthalocyanine catalysts outperformed their porphyrin analogues, underscoring the impact of the second shell coordination environment on the activity and stability of the catalysts. Density functional theory (DFT) calculations revealed that Fe-based catalysts facilitate stronger π-back bonding to *NO, reducing the thermodynamic barrier for NO reduction, which is typically a rate limiting step in the NO3– reduction mechanism. In situ X-ray absorption spectroscopy (XAS) coupled with post-mortem ex situ transmission electron microscopy (TEM) and X-ray diffraction (XRD) showed that FePc/CNT retained Fe–N coordination at potentials as negative as −0.8 VRHE, whereas the metal centers of the other catalysts were reduced into metallic clusters at potentials more negative than −0.6 VRHE. We attribute the enhanced stability and selectivity of FePc/CNT to its local coordination environment. By integrating experimental and theoretical insights, this work elucidates the impact of metal identity and the local atomic environment that synergistically governs the electrocatalytic performance and stability.